Use all configurable colors in TodoWidget

Use business days recurrence in column ui

Previously only priority color was rendered. Now all contexts, projects,
links and metadata should show theirs colors.

Execute commands on multiple items + repeat command

This required to slightly change UIApplication._execute_handler so it
accepts now also a todo id as an argument.

Introduce proper TodoWidget.mark and TodoWidget.unmark public methods so
we don't have to expose _markup.

Use rahter 'mark' and 'marked' instead of 'pending' here and try to use
verb-based function names wherever possible.

Also some minor code-style changes suggested by @bram85.

Repeats last topydo command called from within `TodoListWidget.keypress`
(won't repeat commands called from cli).

Default mapping: '.'

It does 2 things:

1. Clears marked statuses for todo items
2. Resets keystates in all columns

Conflicts:
	README.md
	test/test_list_format.py
	topydo/cli/CLIApplicationBase.py
	topydo/lib/Config.py
	topydo/lib/Utils.py

Don't crash on shlex.split error

1. 'm' can be reconfigured to any other key (action: 'mark').
2. Using 'mark' on already marked todo item will unmark it.
3. 'esc' key will clear whole selection.
4. Command will be executed on **all** marked items in **all** columns.

Show some meaningful error messages instead.

Fixes #107

Add recurrence and postponing by business days

Inspired and also supported by simpletask[1].

`rec:1b` will recur after 1 business day and `rec:5b` after one full
business week. Negative values are supported.

This feature doesn't of course take holidays into account.

[1]: https://github.com/mpcjanssen/simpletask-android/blob/master/src/main/assets/index.en.md

However, this still doesn't work because the configuration is overridden
due to the color=0 hack. Flags like -t and -d will work as soon as the
color setting override is gone.

Thanks to @mruwek for the reminder.

Also removed some unreachable branches from the code.

The color functions now return a neutral/default color when
empty/garbage colors are entered in the configuration file.

This addresses issues #69 and #70.

The -C flag takes 0, 1, 16, 256, auto, yes, true, no, false as possible
values.

If auto, then colors are enabled unless the output is not connected to a
TTY and is likely not to understand ANSI escape codes. 'auto' is the
default value.

The config().colors() function either returns 0, 16 or 256 for the
possible colors that can be displayed.

These values are also recognized by the colors option in the
configuration file.

This fixes issue #111.

This color class abstracts away all the possible representations of a
color in various UIs. These are color objects:

Color(9)              # red
Color('red')          # red
Color(196)            # red
Color('NEUTRAL')      # neutral (resets attributes)
Color()               # no effect

The configuration stores Color objects now, code that requires colors
doesn't have to worry about integer representations of colors.

One semantic change: when an empty color is passed as a priority color,
it will resort to an empty color instead of the neutral color. In
practice this should have the same visual effect. Test case
test_priority_color4 is adapted for this.

This commit is the result after an initial refactoring of the color code
(squashed its history into this one). @mruwek also contributed to this
refactoring, thanks for the help and the ideas to get to this point.

Allow iteration of TodoList items.

Thanks @mruwek for mentioning.

Introduce keymap for column_mode

It's purpose is to show entered keystrokes during entering key-combos.
It's positioned at the bottom-right corner (similarly as in vim). Many
thanks to @bram85 for the idea.

Also fixed one nasty bug resulting in getting stuck in keystate. It
occured only when keystate had also corresponding pending action and
user typed something other than expected by the keypress(). Now any
non-sensical input will result in resetting keystate.

This way user can type offsets and priorities at ease without rush.

From now on user have the choice whether he/she wants the command linked
to key output anything or not. Commands prefixed with 'cmdv' will output
to terminal, while those prefixed with 'cmd' will remain silent.

Example:
========

```ini
[column_keymap]
x = cmd do {}
ls = cmdv ls
ga = cmdv ls -i {}

Credits to @bram85 for the idea.

Special keys in config should follow similiar syntax to that used in
vim:

left arrow = `<Left>`
right arrow = `<Right>`
page down = `<Page down>`
home = `<Home>`
esc = `<Esc>`
F4 = `<F4>`
Ctrl+s = `<C-s>`
Meta+k = `<M-k>`

Examples:
=========

```ini
[column_keymap]
<Left> = prev_column
<Right> = next_column
<Esc>k = home
<C-s> = tag {} foo 1
<M-s> = tag {} foo
```

Action names and default config for them:

```ini
[column_keymap]
0 = first_column
$ = last_column
h = prev_column
l = next_column
A = append_column
I = insert_column
E = edit_column
D = delete_column
Y = copy_column
L = swap_left
R = swap_right
```

User can now specify key-shortcuts in main topydo config file under
'column_keymap' section in form of:

`<SHORTCUT> = <ACTION>`

Two main types of action are supported:
- built-in (one of: 'home', 'end', 'up', 'down', 'postpone', 'postpone_s'
  and 'pri')
- topydo commands (aliases included). Prefixed with 'cmd'. Commands to
  call on selected todo item should contain '{}' placeholder to mark its
  place in final command call in similiar fashion as in aliases
  definitions.

postpone, postpone_s and pri shortcuts are sort of prefixes for
arguments for respective topydo commands triggered on selected todo item:
- `postpone<COUNT><PERIOD>` will translate to `cmd postpone {}
  <COUNT><PERIOD>`. postpone_s will do the same but with '-s' flag
  added.
- `pri<PRIORITY>` will translate to `cmd pri {} <PRIORITY>`

Default config as an example:

```ini
[column_keymap]
gg = home
G = end
j = down
k = up
d = cmd del {}
e = cmd edit {}
u = cmd revert
x = cmd do {}
pp = postpone
ps = postpone_s
pr = pri
```

- pp23d will postpone selected item by 23 days
- ps1m will postpone selected item (threshold date included) by 1 month
- prz will set priority of selected item to (Z)