assets/Html: Replace commonly known templating syntaxes that produce inv...

...alid html with internal placeholders while working with the parsetree. Inject original content back when using HTML.text getter

This adds support for underscore templates, jsp, asp, php. Any language using `<% ... %>` or `<? ... ?>` for inserting stuff inside html. Still no guarantees that the resulting html is valid. It's up to the user to avoid stupidity here.

@papandreou This is a heads up. I have no tests yet to verify, other that a successful build of some todomvc apps. I'll work on tests tomorrow, but I'd like some feedback on this change since it feels a bit dirty to me.