Add support to decrypt to Opus format

[gray-open]

I wanted to copy my audiobooks to my Android phone and listen to them there with an audiobook app. Trouble is, my books were over 100GB. Also the Android app I was using needs audiobooks in separate folders.

Some new options could be added to the decrypt plugin:

--opus     - enable description directly to Opus audio format rather than m4b
--folders  - output audiobook files into <author>/<title>/ sub-folders
--bitrate  - optionally tweak the Opus output bitrate 
--jobs n   - support multiple parallel jobs when decrypting

So to decrypt all the audiobooks in the folder "encrypted" and output in Opus (defaulting to 16kbps), processing 5 at a time:

audible -v error decrypt -d audiobooks --opus --folders --jobs 5 "encrypted/*"

Output is around 5x smaller with 16kbps Opus.

[devnoname120]

For your folders issue you could alternatively just use a better audiobook app, for example Sirin: https://play.google.com/store/apps/details?id=com.sirin.android