Transcode the stream if ffmpeg-python is available

[bct]

If the ffmpeg module (https://github.com/kkroening/ffmpeg-python) is available, transcode the output of /rest/stream to respect maxBitRate.

If ffmpeg is not available, it will fall back sending the raw file.

I also changed the code for sending the raw file to use flask.send_file - this is supposed to be more efficient.

I've tested this manually, with & without ffmpeg. I'm not sure this is the most effective way to transcode, but it's been working for me.

[BinaryBrain]

Awesome idea and thanks for the PR!
I found a bug when transcoding MP3 files containing cover art.
I also added an option never_transcode: False that you can turn to True to deactivate transcoding (I use it on a Rpi and 30s to encode a mp3 file is a bit much for me).
I added this in #10 .