[rfc] enable opus codec in ffmpeg

this was requested in irc. 

so far I have build-tested for linux (ubuntu and openelec) only. the linux part is quite trivial, but since I am not much familiar with xbmc's own buildsystem - this needs some love from osx/android/rbpi platform devs (opus in tools/depends/target), and (maybe) changes to jenkins

/cc @wsnipex @fernetmenta to check for stupids.