added support for srt

[JoeSchiff]

This will solve: #1503

I would like to set up a test for this, but I don't know how to start streaming a video in pyav. I was able to create a network live stream with this:
ffmpeg -i local_file.mp4 -c:v libx264 -f mpegts 'srt://127.0.0.1:40052?mode=listener&latency=2000000'

I then opened the network stream with:

path = "srt://127.0.0.1:40052?mode=caller"
container = av.open(path)

Pyav decoded the network stream and everything seemed to work.

If anyone knows how to create a network stream in pyav, I could write a test for this.