Add audio channel mapping.

Add audio channel mapping.

This uses meta properties on the producer:
meta.map.audio.<N>.channels=<integer>
meta.map.audio.<N>.start=<integer>
where 8 > N >= 0.
The consumer loops over N, consumes meta...channels and outputs them
from channel meta...start. The channel index starts at 0. For example,
if the first audio track of the clip is stereo English and the second
audio track is stereo French, then this will swap them:
$ melt someclip audio_index=all meta.map.audio.0.channels=2
meta.map.audio.0.start=2 meta.map.audio.1.channels=2
meta.map.audio.1.start=0 -consumer linsys_sdi.

However, the last pair of meta properties in this example are actually
optional. The algorithm outputs the remaining channels at the start
channel you specify. But since getting an unspecified property yields 0,
the last meta properties is unnecessary as well.