HLS-34: Audio Bitrate Scaling

It's always been a feature of the HLS spec, but isn't something I've really had a use for.

A user has asked in Github #33 for a switch to set/scale audio bitrates.

This could be implemented in one of 2 ways:

- You can specify an audio bitrate alongside a video bitrate. The audio track in segments with that video bitrate would have their audio at the specified rate (or native if not specified)
- A new flag (for sake of argument -a) is added and works a lot like -b. You pass in the audio bitrates you want (e.g. -a 96,128,384). Audio track is stripped from segments and broken out into separate audio streams and declared in the manifest

The former is much easier to implement, and gives the operator more control.

The second is harder to implement, but gives the player more choice.

Having separate audio tracks also lays the groundwork for possible future enhancements - like (for example) specifying alternate audio tracks (e.g. German audio instead of English etc).

It'd also mean we could (potentially) implement the ability to ignore video and only output audio streams (allowing the script to be used for things like adaptive radio streams).