Related to issue #194.

Suggest to use pip3 over pip

`topydo revert ls` will now show all saved backup points with position
number (starting with 1), date and time of relevant action, and its
label.

`topydo revert 2` will revert to a point **before** the backup point
marked as 2 in `topydo revert ls` (i.e. it will go 2 changes backwards
or phrasing more naturally: revert last 2 changes).

Warning: changes between between current state and desired backup point
are also deleted to avoid false assumption that our backups can be
treated as some sort of atomic "patches".

When user disabled archive feature after he made some backups, don't try
to restore archive during `topydo revert`, as there is no file to write.

self.todolist may not be present; it could be None. This is the case
when the list formatter is instantiated from a TodoWidget in the column
UI, which passes None for the todo list.

The overrides are necessary to make the multicommands/widgets behave
well.

'test' is not really a library that has to be imported

Among others, moved methods by turning them into (inline) functions or
static methods.

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.