Instead of enforcing the user to know and understand caps to describe
the encoding format, let him use element factory names directly.
This also makes it possible to ensure that a specific encodore/muxer
is used instead of letting the ranking system do it.

It is now possible to describe an encoding format simply specifying:

  matroskamux:x264enc:vobisenc

Factor out functions in the parsing, cleaning up the whole thing.
Update documentation.