Combine video and audio into one ffmpeg process

[Brendonovich]

Don't review until #45 is merged

Combines the audio and video ffmpeg arguments into a single command that receive each stream via named pipes. Video segments are now uploaded to ${userId}/${videoId}/segment/, and the default audio selector is always enabled for MediaConvert. Not sure if that's all been done properly, but the ffmpeg stuff in media.rs should be good.

/claim #24 Closes #24

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[richiemcilroy]

This is awesome @Brendonovich, great work!

[Brendonovich]

/claim #24