added html element visitor

Hi, 
thanks for the tip on the mailing list, I added an Html element vistor, you can merge it if you like, any feedback welcome. 

I've tested with the example in the readme

I build with Haxe3 so I changed a few things but the particular class I share should build fine with Haxe2.

I can write a sample if you want.

Good work by the way !