More intelligent audio track selection

[jbillo]

Intelligently select audio track based on format and channels:

• Consider language preference above all - some source files may contain 5.1 English and 2-channel AAC preference
• Consider commentary/other tracks that may be in 2-channel AAC and the same language

Theoretically if we can resolve these, use the following selection criteria:

• If an MKV file contains an AAC track that matches the channel preference or profile requirement, select it over an AC3/DTS one (don't recode audio)
• Fall back to more easily convertible formats (prefer AC3, then DTS) over ones that are known to choke mplayer