extractor: youtube: Allow format number as output template.

Since many video formats map to the same extension, we get problems when
trying to download more than one format of the same audio/video with the
same ID.

This commit introduces a numeric version of the template of the video
format, which can help users who start downloading with a format (say, 720p
mp4), stop and then resume with another format (say, 360p mp4). See:

* http://bugs.debian.org/689294
* http://bugs.debian.org/643831

An output template like '%(title)s-%(id)s.%(format_num)s.%(ext)s' would,
unmistakenly, solve such problems.

This is also helpful to know what format was effectively downloaded when a
user expected something else (like, for instance, in issue #1578 or many of
its duplicates).

Another application of this is in separate downloads of audio video (as in
issue #1612).

Signed-off-by: Rogério Brito <rbrito@ime.usp.br>