Related to issue #194.

Suggest to use pip3 over pip

The defaults / fallbacks are such that it cannot exceed 6 characters.

I don't like typing numbers, and would like to limit the characters to
be those that are easily reachable with a Dvorak keyboard layout.

When using numerical IDs, the ID width is padded to the log10 of the
todo length.

When using textual IDs, todo lists of 466 items or shorter pad to 3
characters, otherwise 4 characters.

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).