Support for multiple media streams

[m4reko]

I needed to get RTSP with multiple media streams working in native Python so I made an attempt to extend this library with that functionality. Works well in my implementation but I have not been able to run the tests locally or implement new ones for my extension. If you find it promising I can attempt that.

I'm not sure if the implementation is completely correct according to the RTSP protocol either. I'm pretty new to this stuff...