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

Actually it's a two letter shortcut. Second letter stands obviously for
priority.

Example:
- 'rc' will trigger `topydo pri TODO_ITEM C`

Shortcut calls postpone command and accepts similiar format as original
`topydo postpone`.

Examples:
- pressing 'pd' or 'p1d' will postpone selected task by one day.
- 'p3y' will postpone task by 3 years.
- 'p12w' will postpone task by 12 (twelve) weeks.

Introduce config file for column layout

Column-ui will look for columns configuration in:

1. system-wide configuration:
/etc/topydo_columns.conf

2. user dir:
~/.topydo_columns
~/.config/topydo/columns

3. current dir:
.topydo_columns
topydo_columns.conf
topydo_columns.ini

Each column should have its own INI style config section:

[FooBar]
title = Foo
filterexpr = Bar
sortexpr = desc:prio
show_all = 1

If any column option is missing, default value will be used.

Also move home_config_path() out of the Config class. It didn't use
'self' argument and this way we can reuse it other places.

Support backups (and `revert`) in column ui

Thanks to @mruwek for investigating.

Add `topydo columns` subcommand to launch column-ui

Switch column-ui to 256 color-mode

Drop python2 in column-ui and fix pylint errors

Fixes "not-callable" pylint errors.

A list of strings should be passed instead of a string.

Thanks to @mruwek for reporting.

When a filter with a creation date matches, don't process expression
with the remaining filters.

Addresses issue #86 which requested filters on completion date. The
following tags are recognized:

* create
* created
* creation
* complete
* completed
* completion

They are specific ordinal tag filters, except that these tags don't
exist in a todo item.

Filtering on completion date does not work when completed items are
automatically archived. Archived items are not read when invoking `ls`,
so the done.txt file should be read as the main file instead:

    topydo -t done.txt ls complete:today

Also in this commit, decouple ExpressionCommand from Filter class a bit.

The expression class shouldn't bother which expressions belong to which
filter classes, let the Filter module handle that.

Create placeholder for args in aliases

With this change there is now possible to forward args supplied on CLI
to desired placed in real command.

Example config:

[aliases]
star = tag {} star 1
unstar = tag {} star

Example usage:

`topydo star foo` will resolve to: `topydo tag foo star 1`
`topydo unstar foo` will resolve to: `topydo tag foo star`

Alternate for `top` command

Uses new %P format and -N parameter.

`ls -N` will display one 'screen-ful' of lines (the number of lines in the
console, less 2).

%P will display the priority if one is given, or print a 'placeholder' space
if none is present.

Priority command: allow lowercase priorities to be input