AAC audio: MP3 transcoding as backup option

[dinkypumpkin]

For anyone interested, I've made a version of get_iplayer with an option to transcode AAC audio to MP3. I started doing this when flashaudio began disappearing from iPlayer, and now I've piggybacked it on Nick Ludlam's changes for AAC->M4A remuxing. It can found at:

https://github.com/dinkypumpkin/get_iplayer/blob/aactomp3/get_iplayer

If anyone would like to test it, please have a go.  To force MP3 transcoding, add '--aactomp3' to the command line or 'aactomp3 1' to the options file.  YMMV, no guarantees, no refunds.

I know transcoding is not a great option given that it reduces quality and can crawl on a slow machine, but I think it might be useful to have this alternative built into get_iplayer.  It would be a backup option for those players that really can't deal with AAC or M4A, and since it would be a positive user choice it wouldn't interfere with the default behaviour.