`connection.processSdp` method added.

Now you can process SDP for stereo audio or opus/vp8 parameters or
application-level bandwidth management.

```javascript
connection.processSdp = function(sdp) {
    // process
sdp here; e.g.
    // sdp = changeOpusParameters(sdp);

    // you
must return "sdp" back.
    return sdp;
};
```

This feature gives you
full control over SDP-modification. You can even use something like
[SdpSerializer](https://github.com/muaz-khan/SdpSerializer) to modify
the SDP.

Note: This function will be called by RTCMultiConnection code. It
accepts single string-argument where SDP is passed. There is no clue of
the type of sdp, though. You MUST return "sdp" back using "return"
statement.