Allow merging formats (closes #1612)

Multiple formats can be requested using `-f 137+139`, each one is downloaded and then the two are merged with ffmpeg.
Merge remote-tracking branch 'jaimeMF/merge-formats'
[collegehumor] Support multiple formats (Fixes #2092)

Unfortunately, we lose a part of the description in the new JSON format, but that's still better than a non-functioning URL.
[collegehumor] Add support for age_limit