DASH muxer

Hey @Vanilagy,

Now that HLS is officially supported I wanted to open a ticket for DASH support. There's a lot of overlap, probably only the way that the manifests are written differs.

Right now our recorder uses ffmpeg in the browser to do DASH muxing, it means we can support all browsers, ones that can do h264/vp9/vp8 etc in the same playlist.

Main reason: DASH supports vp9/vp8 as well, HLS officially does not, and some players won't play it.

Let me know if you're interested in this! I can provide examples or a PR if you'd like.