When a subcommand finishes, _post_excecute() is called for sorting,
archiving and writing the file. But sorting is also a subcommand, which
in turn also calls _post_execute(), etc, etc.

So let the caller of _execute() decide when to call _post_execute.

This fixes issue #46.

Thanks to Jacek for pointing out.

Safe 16 base colors

- Colors specified by name will return safe ANSI codes.
- Colors specified by number will return ANSI code from xterm 256 color
  chart.
- Defaults are also *safe*.

This is a revert of commit fbed527f, which resulted in too many items
after a project/context being highlighted.

Instead, don't print a NEUTRAL_COLOR code at the end of the todo, but
only when we finish applying all the color codes. This makes sure that
the regexp to match a project/context also works at the end of a line.

Also removed some logic to remove multiple sequences of NEUTRAL_COLOR at
the end of the line, this is not necessary.

@\033[0m gets matched by '\B(\+|@)(\S*\w)', and as a result, the @ would
be colored to the context color.

Probably an indenting mistake.

Add possibility to configure colorscheme of topydo

The completer would be instantiated before the todo list was loaded from
the file. Now, the completer is instantiated every time the todo list is
loaded.

+ -> @

- Add new module `topydo.lib.Colors` and class `Colors` for handling
  output colors and their ANSI codes.
- Create new config options with colors for priorities, projects,
  contexts, metadata, and links and place them under "colorscheme"
  section. When certain config option is absent default color code will be
  applied.
- Empty values:
    ! priority_colors will fall back to NEUTRAL_COLOR
    ! project_color, context_color, link_color, metadata_color will
      return empty string, so priority_color (if configured and applicable) of
      specific item should be used
- Return NEUTRAL_COLOR if supplied color number is out range
- Support user-friendly color names in config. At least for 16 base
  terminal colors.

Instead of creating a subcommand for every possible output format we're
going to think of in the future, just let ListCommand handle the output
format by connecting to the right format printer.

In this commit, the 'ical' subcommand is deprecated, to be removed after
the next release.

The 'json' subcommand has been removed altogether, it existed for only a
few hours.

The todo.txt file can be modified while the prompt is running. Make sure
to reload the file when the mtime has changed since the previous load.

This addresses the issue that changes after the 'edit' subcommand were
not visible when returning to the topydo prompt (issue #33).

Also, other changes done while the prompt is running will be picked up.

A safety measure was taken to refuse executing a command when the
todo.txt file was modified while the prompt was visible (i.e. the user
was busy typing a command). By doing so, subcommands such as 'do' and
'del' will only operate on the actual state of the todo.txt file, not
some previous state still in memory.

Fixes #43.

topydo was a tad slow, because all modules of all subcommands were
loaded with each invocation of topydo. Particularly, the ical subcommand
was quite slow because it loads modules from the icalendar package.

Add todos from file with '-f' flag.

1. Move _preprocess_input_todo adn _postprocess_input_todo inside
_add_todo.
2. Remove self.todo

Use unittest.mock wherever it's available

When obtaining a configuration object, it is now possible to pass
certain values to be overridden no matter what settings were made in the
configuration files.

This is used to let the -t and -d flags (location of todo.txt and
archive respectively) override anything that is mentioned in
configuration files.

This fixes a bug where the edit subcommand did not respect the -t flag,
because it was simply reading the location from the configuration file.
Now the configuration is overridden when -t is passed.

Thanks to Jacek for reporting the broken 'edit' command.

Fix unicode in ical.

Addresses Unicode issues under the umbrella of issue #21.

Two possible future improvements regarding editor selection:

* configure editor through configuration file
* set editor through a flag, e.g. topydo edit -p gedit