====== wrtc_cmd ======
**C → S → C {type: 'wrtc_cmd', from:..., to:..., cmd:...}**

wrtc_cmd – user defined command sent to other client. wrtc_cmd can contain any data a client want to transmit to another client. The data is transmitted using the RTCgate signaling layer.

===== Arguments =====
  * from - address of client sending the command
  * to - (optional) address of client to receive command. If missing command will be sent to partner at other end of call.
  * cmd - data to be transmitted. The data should be as small as possible, preferably below 1KB. Recommended to use JSON structure with cmd and arguments.