Add Support for RTSP attributes (a=contorl, a=range)

[mateuszkj]

https://tools.ietf.org/html/rfc7826#section-22.15

[na-g]

@mateuszkj thanks for the patch! Could you comment on your use case for this library to give us some context for the patch?

[mateuszkj]

@na-g I had to write RTSP proxy with audio permissions. User without audio permission would receive SDP without audio codec (so VLC will play only video without audio).

I parsing SDP and then formatting (with or without audio codec), but original webrtc-sdp would drop attributes a=control and a=range witch are required by RTSP.

In Summary, my patch prevents webrtc-sdp from dropping these attributes while paring/formatting and also I can access value of these attributes.

[xuguangyuansh]

Our library supports rtsp to fetch the stream, and then transfer it to webrtc for playback, right? Didn't see how to configure it? I use it for security/surveillance.