[WIP] Custom namespaces

An extended implementation of https://github.com/Masterminds/html5-php/pull/44 that supports also custom namespace prefixes and XML style namespaces.

See here to see an example: https://github.com/goetas/html5-php/blob/custom-namespaces/test/HTML5/Parser/DOMTreeBuilderTest.php#L99


This needs to be documented somewhere, so that if someone uses the parser with their own serializer, they can still handle this case. Maybe this could go in the readme section about "known issues".