Support backups (and `revert`) in column ui

Thanks to @mruwek for investigating.

Add `topydo columns` subcommand to launch column-ui

Switch column-ui to 256 color-mode

Drop python2 in column-ui and fix pylint errors

Fixes "not-callable" pylint errors.

A list of strings should be passed instead of a string.

Thanks to @mruwek for reporting.

When a filter with a creation date matches, don't process expression
with the remaining filters.

Addresses issue #86 which requested filters on completion date. The
following tags are recognized:

* create
* created
* creation
* complete
* completed
* completion

They are specific ordinal tag filters, except that these tags don't
exist in a todo item.

Filtering on completion date does not work when completed items are
automatically archived. Archived items are not read when invoking `ls`,
so the done.txt file should be read as the main file instead:

    topydo -t done.txt ls complete:today

Also in this commit, decouple ExpressionCommand from Filter class a bit.

The expression class shouldn't bother which expressions belong to which
filter classes, let the Filter module handle that.

Create placeholder for args in aliases

With this change there is now possible to forward args supplied on CLI
to desired placed in real command.

Example config:

[aliases]
star = tag {} star 1
unstar = tag {} star

Example usage:

`topydo star foo` will resolve to: `topydo tag foo star 1`
`topydo unstar foo` will resolve to: `topydo tag foo star`

Alternate for `top` command

Uses new %P format and -N parameter.

`ls -N` will display one 'screen-ful' of lines (the number of lines in the
console, less 2).

%P will display the priority if one is given, or print a 'placeholder' space
if none is present.

Priority command: allow lowercase priorities to be input

Don't just consider the first p tag value in a todo item, consider all
of them.

Given these dependencies:

A id:1
B p:1
C p:1 id:2
D p:1 p:2

A -> B
A -> C
A -> D
C -> D

The relation A -> D is superfluous (A -> C, C -> D). However, the p:1
tag in D would not be removed, because relation 1 still exists (A -> B).

Therefore a new function is added that identifies p: tags which have no
corresponding edge in the internal dependency graph (rather than looking
at the value of the edge).

Conflicts:
	setup.py
	topydo/lib/PrettyPrinterFilter.py

This assertion was missing in commit 6bb804dc.

To complete a todo item that you finished yesterday, run

    topydo do -d yesterday

Or

    topydo do -d -1d