#83 MP3 encoder plugins

[Jarl van Katwijk]

Both the mp3 encoder plugins have a few bugs\limitations:

- the one using the bundled mp3lame library cant handle
a 2nd audio channel: both the encoded audio channels
are ruined in that case
- the plugin that loads a standalone mp3lame
installation can do multiple audio channels, but only
produced correct audio at 128kbit encoding

I hope to find some time to supply patches, but maybe
you avifile guys can fix this without much research;)