ExternalOutput h264 support

Hello All,

I’m currently working on adapting the ExternalOutput to support outputting h264-encoded videos. I also have a currently working audio transcode set up that would hopefully be useful for others, so I will look forward to PR’ing these.

All of this is to say that it would be good for me to talk to someone more involved with Licode who knows more of the stack to verify my work. I’ll try to attend the weekly office hours this week, but I didn’t see anyone there last week (The time is specified as 16:00 CEST - is this now 16:00 CET?)