It will mark all todo items contained in currently selected
TodoListWidget. By defualt it is mapped to <C-a> (Ctrl-a).

Add possibility to trigger actions after archiving

Each command can now execute action defined in its own
Command.post_archive() method after archiving action is done. Hook for
`do` and `del` is also included in this commit.

This should fix #139

Plain 'help' would crash the column UI, as would the usage of an invalid
command in an alias.

This case should be handled properly, as the CLI and Prompt mode do.

For this purpose, the generic help text was extracted, such that it can
be printed in column mode (and also in prompt mode, while I was at it).

The LimitFilter should be performed at the very last step. Instead of
complicating ExpressionCommand and ListCommand to cough up a well
ordered set of filters, order the filters when updating the view. Each
filter is assigned an order number, where filters with a higher order
are applied later.

This fixes the output when using the ls -n flag, where the truncation
appears before the hide tag filter. This results in possibly less items
to be printed than specified (or worse: nothing is printed).

Fix config duplicates and catch ListFormatParser errors

Introduce 'before' and 'after' keywords to `dep ls`

Lack of printing empty result was especially confusing in column UI
(console wasn't opened and user could think that command wasn't
executed).

Use last definition of an option instead. Section duplicates are also
supported from now.

Example:

'''
[column_keymap]
a = foo
a = bar

[column_keymap]
b = foobar
'''

is equivalent of:

'''
[column_keymap]
a = bar
b = foobar
'''

From now on:
`topydo dep ls before 1` gives the same output as `topydo dep ls 1 to`
and:
`topydo dep ls after 1` gives the same output as `topydo dep ls to 1`

Allow actions on multiple ids without MultiCommand

This will prevent from leaving TodoList and todo.txt file in
inconsistent state whenever first Command.execute() inside Transaction
succeeds and any subsequent fails for whatever reason.

This provides that each backup description will still inform user about
each command that was executed. When transaction of commands was
executed user will get printed list of commands separated by ';'.

Example:

Successfully reverted: append 1 FooBar; append 3 FooBar; append 6 FooBar;

This feature means that commands like `tag`, `append` or `dep` can work
now with multiple (marked) todo items in column UI.

Lookup mechanism:

1. Check whether an editor was given with the -E flag:

     topydo edit -E vim

2. Use the value of $TOPYDO_EDITOR
3. Use the value in the configuration file:

   [edit]
   editor = vim

4. Use the value in $EDITOR
5. Use 'vi' if the above fails.

This makes it easier to invoke an editor as vim with additional
parameters, for instance by adding the full todo.txt file to the
completion options (see the tip in issue #164). This trick was added to
the configuration file as a comment.

Prefer filterexpr if there is no title in config

Use 'Yet another column' only if there is no title and filterexpr is
also empty. This behavior is in accordance with change introduced in
fc154914.

Suppose we have the todo item:

    (C) Some item key:value1 key:value2

And then I do an ls:

    topydo ls -x key:value2

Before, this would yield an empty result, which is undesired. Therefore,
only apply ordinal tag filtering when a tag appears exactly once in the
todo item. This is a simple approach, and special operators >, >= etc
will not be applicable:

    topydo ls -x key:>value1

will no longer work. The result could be ambiguous: filter out when none of
the tags match, or only some of them?

Don't crash on FileNotFoundError with `add -f`

Show descriptive error message instead.

Relative dates were already interpreted for due and start tags, but you
can also enforce it for other tags by passing the -r flag to the tag
command.

* Extended the feature list.
* List the UIs.
* Added a screenshot of the column UI.
* Dropped information on dependencies, the user should not worry about
  this with pip. Or just look at setup.py.