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.

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

'A' appends a new column.
'I' inserts a new column before the current column.

'N' was dropped.

This reverts commit 3513603a.

TodoListWidget actually uses this variable to associate the widget with
the Todo item.

Thanks to @mruwek for the heads up.

Main concept and great part of implementation derived from
[pazz/alot][1].

[1]: https://github.com/pazz/alot/blob/0.3.7/alot/widgets/globals.py#L76

Gets rid of creation date and hidden tags that consume precious space.

Temporarily disable color output in column-ui's console

We don't want raw ANSI codes there. We'll enable it back again after
providing proper coloring solution.

New key-shortcuts for column-ui

Thanks to @bram85 for the idea.

Fix crashes when last item in column disappeared

Almost identical to `p`:
- `ps3d` will call `topydo postpone -s TODO 3d`
- `psy` will call `topydo postpone -s TODO 1y`

_command_on_selected: accepts string containing command (with potential
arguments) to execute and replace field (for further substitution with
todo id) as an argument.

There was uncaught IndexError when dealing with focus.