Add media-stream setting

Add hidden setting to enable media-stream support (webrtc) in case webkitgtk supports it.

@mcatanzaro: This would really help me a lot. Instead of building Epiphany again and again just to have webrtc beside working on new features. As this is a hidden option, defaulting to false i think it is fine to go in. I've furthermore added a description explaining that a feature enabled webkitgtk build is necessary.