Much more powerful filtering of priorities

Now it is possible to do things like:

`ls (>C)`  # gives items with A-B priorities
`ls (<=D)` # gives items with D-Z priorities
`ls (!E)`  # gives all items except those with E priority (also possible
           # with `ls -- -(E)`

and maybe most importantly:
`ls (<Z)` # gives items without priority

PS Don't forget about escaping parentheses when calling from CLI

Move core functionality of Filter.OrdinalTagFilter to it for possible
reuse in future.

This was the result of the backward incompatible change in
prompt-toolkit 0.47.

This one supports the `complete_while_typing` flag in boolean form.

Windows cmd 2 -- add color to output on Windows

Fixes #32

See #32

Insprired from the Simpletask client for Android.

https://github.com/mpcjanssen/simpletask-android

See:
https://bitbucket.org/logilab/pylint/issues/487/configparserget-return-value-gets-treated

Fixes #20.

Tags with a value of length 1 were not highlighted with the metadata color.

Compare:

http://pythex.org/?regex=\b\S%2B%3A[^%2F\s]\S%2B\b&test_string=star%3A1%0Astar%3A12&ignorecase=0&multiline=0&dotall=0&verbose=0

http://pythex.org/?regex=\b\S%2B%3A[^%2F\s]\S*\b&test_string=star%3A1%0Astar%3A12&ignorecase=0&multiline=0&dotall=0&verbose=0

If there was a todo with 'foo:somestring', it wouldn't be returned with
the command above, because somestring couldn't be converted to a date or
an integer, and therefore the OrdinalTagFilter just returns False.

As a final fallback, use a GrepFilter match to do the string comparison
such that the todo item(s) with this key:value combination will be
returned.

Priority Aa or AB is (still) not allowed.

Conflicts:
	README.md

The previous commit had the archive condition on the wrong place,
resulting that todo.txt files were not written out at all if archiving
was disabled.

Also introduce the (overall) -a flag to disable auto archiving. -a comes
from the original todo.txt CLI.

It would have been nicer to have DoCommand support the -a flag, however,
DoCommand doesn't know anything about archiving and I would like to keep
it that way.

Addresses issue #42.