Allow specifying custom libdatachannel configuration options

[Faless]

This includes:

• enableIceTcp
• enableIceUdpMux
• portRangeBegin
• portRangeEnd
• mtu

e.g.

var pc := WebRTCPeerConnection.new()
if OS.get_name() != "Web": 
	pc.initialize({
		"libdatachannel": {
			"enableIceUdpMux": true,
			"portRangeBegin": 4343,
			"portRangeEnd": 4343,
		},
	})