Stream copy from template

Addresses issue #730 (closed) and #507 (closed). When adding a stream via a template, look up the encoder via the template codec's id, consistent with Transcoding.c in the ffmpeg source: https://github.com/FFmpeg/FFmpeg/blob/66deab3a2609aa9462709c82be5d4efbb6af2a08/doc/examples/transcoding.c#L149

Why all the new enums?

Sorry, just saw this. My GitHub notifications are a mess.

I had to add them because of the declared types in this method: https://github.com/PyAV-Org/PyAV/blob/1213f02dc28a14aa803753a8b57f9eae9e07604a/include/libavcodec/avcodec.pxd#L533

added changes requested label

restored source branch github/fork/alexhutnik/stream-copy-from-template