Support audio 🔊

We don't currently have support for audio in our webrtc streams. This is due to some docker-os missing pieces (alsa loopback device not found). We should investigate further what is needed for __in container__ audio, and implement it.

```
{"level":30,"time":1562170079310,"pid":70,"hostname":"3195a53213dc","msg":"Browser: selected device {\"id\":\"screen:0:0\",\"name\":\"Entire screen\"}","v":1}
ALSA lib pcm.c:2495:(snd_pcm_open_noupdate) Unknown PCM loopback
[70:0703/160759.316693:ERROR:alsa_util.cc(204)] PcmOpen: loopback,No such file or directory
{"level":50,"time":1562170079318,"pid":70,"hostname":"3195a53213dc","msg":"Browser: failed to boot: NotReadableError: Could not start audio source undefined","v":1}
```