allow fullsnapshot capture to be added along with constructor

[eoghanmurray]

Allow .on to be chained directly after the constructor so that event handling for the first full snapshot can be added straight after constructor, and that handler will receive the first automatic fullsnapshot build (#216)

Could do with some documentation so you can see:

	var rrweb_replayer = new rrwebReplay.Replayer(
		recording
	).on('fullsnapshot-rebuilded', function(fullSnapshot) {
		// do something with first build
	});

[Yuyz0112]

LGTM

Thanks