youtube-dl: make a python package

This allows youtube-dl to support all versions of python, and be built
correctly even when the python version is overridden by the user.

An alias to youtube-dl has been added which points to the default python
version's youtube-dl.

Also, ffmpeg has been made into an optional dependency. If ffmpeg is set
to 'null' then it will not be built and there will be no wrapper for it
either. An extra package, youtube-dl-light, is the same as youtube-dl
but without the ffmpeg dependency.