Typing numbers is less convenient than typing alphabetic characters (at
least if you're a blind typer). So before resorting to numbers, try to
generate IDs with alphabetic characters only. Only when the ID space
runs out, introduce numbers.

Now the screenshot is visible immediately when you visit the website.

For other tags containing date values, try to print the relative
humanized date as well. If the tag does not contain a date in YYYY-MM-DD
value, it's normal value is printed.

Fixes #186.

ctrl-a: move cursor to the beginning
ctrl-e: move cursor to the end
ctrl-u: delete from the cursor back to the beginning
ctrl-k: delete from the cursor to the end

- completion box pops out for multiple candidates
- <Tab> and <Shift-Tab> will navigate through the list of candidates
- any other key than <Tab> and <Shift-Tab> will close the box
- completion box is glued to cursor and is trimmed to max 4 lines
- +projects, @contexts, dates and commands (with aliases) are supported

1.Rename completers

topydo.ui.prompt.TopydoCompleter is now topydo.ui.prompt.PromptCompleter
topydo.lib.Completer is now topydo.ui.CompleterBase

2. Reuse CompleterBase code in prompt completer.
3. Sort completion suggestions
4. Introduce completion of due: dates in column completer.
5. Store subcommands for completers in cache (via lru_cache).

Currently it works only with single completion candidate and works for:
contexts, projects, subcommands.

The postpone subcommand would crash when postponing a todo item with an
invalid due date (e.g. due:2017-06-31).

When postponing, the due date serves as an offset. With an invalid due
date, no proper offset can be found. Instead of (silently) falling back
to today's due date, show an error instead.

Moreover, invalid creation/completion dates are also ignored.

%n: Line number
%N: Padded line number
%u: Text-based ID
%U: Padded text-based ID

Addresses issue #184.

With 1.7.1 and higher we can use it in Python 3.6.

This reverts commit 25144172.

Located at https://www.topydo.org/static.html

Introduce new 'mark_all' action in Column UI

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.