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.

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)

The Github editor was trying to be helpful.

Conflicts:
	setup.py
	test/test_list_format.py

This will reload all columns with respect to due dates, start dates and changing importance values.

Initialize it on startup and use that color map, instead of
instantiating it for every todo widget.

There is no need to instantiate the parser for each widget since the
expression remains the same. Should save some CPU cycles for each screen
update, in my case having over 600 todo widgets activated.

This addresses some slugginess that I encounter in the column UI. For
example, when doing an `ls` on the commandline, formerly it would
execute:

* Perform backups;
* Write the todo file (+ archive if applicable);
* Refresh all the columns;

All in all this takes quite a while.

CLIApplicationBase::_post_execute will now reset the dirty flag of the
todo list. Then writing stuff and refreshing columns will only occur
when the todo list is dirty.

Updating columns can still be made smarter. When an action is executed
on a specific todo item, only update those columns that contain that
item, or are related to it through dependencies.

Applying a filter is a primary thing to do for defining a column, therefore put it on top.

The console widget is slightly smaller than the terminal size, so the
column UI should help get_terminal_size a bit to determine the correct
size.

Don't concatenate column layout configs

Use only one layout file. Following precedence is applied:

1. Local (i.e. $PWD/topydo_columns.ini or $PWD/topydo_columns.conf or
   $PWD/.topydo_columns)
2. User's home (i.e. $HOME/.topydo_columns or
   $HOME/.config/topydo/columns)
3. System-wide (i.e. /etc/topydo_columns.conf)

Add history to command-line widget