Allow feeding H.264 into the webrtc wrapper

[floe]

Right now, all encoding is done in the wrapper and only raw video can be consumed; would be more flexible if an option for consuming already-encoded H.264 is available. This is a prerequisite for addressing #31.

NB: in parallel, some scenarios would benefit from not hard-wiring a decodebin into the output, but also allowing to output the encoded stream.