plugin: opus: introducing new codec.

Some codec properties could be changed by rebuild
(e.g. computational complexity, use FEC, etc.). Note, that the frame size
is fixed on 20ms as the rest is not ready to get different value, eventhough
the opus RFC specifies frame size between 2,5 and 120 ms.
opus: use standard include path instead of adding subpath in Makefile
core: use codec's PLC function if it is provided
opus: b/f: support only 1 channel, less debug log