Also make the ReadCloser an Controllable allows to uncouple
the controller implementation from the encoder.
This is not needed for the 2 codec controller already implemented (openh264 and vpx)
but is more future proof in case it required for other codecs.
Resolves https://github.com/pion/mediadevices/issues/114
* Remove codec registrar
* Completely redesign how codec is being discovered, tuned, and built
* Update examples
* Update unit tests