Add WebRTC support using libdatachannel

The WebRTC is exposed via `/video.html` endpoint
and enabled by default as long as h264 stream
is available.

html/index.html

@@ -40,7 +40,7 @@
 		<br>
 		<li>
 			<a href="video"><b>/video</b></a><br>
-			Get a live video (H264) stream.<br>
+			Get a live WebRTC (H264) stream.<br>
 			<br>
 			<ul>
 				<li><a href="video.mp4"><b>/video.mp4</b></a><br> get a live video stream in MP4 format (if FFMPEG enabled).</li>