Custom Tag support and field overhaul

[jhy]

I'm planning on adding Custom Tag support, that will allow various tag properties (white space formating, void, self-closing, etc) to be configured before the Parser is run.

Currently we infer these properties for unrecognized HTML tags (in the HTML tree builder) and for all tags in the XML tree builder.

Would look to have a new Tag instance creator, and a way to provide the Parser with the list of tags. In the case of HTML that could be by modifying the default list (probably extending). In XML it would be the entire set.

These would impact the parse when possible in the HTML5 tree builder as most rules don't come from tag properties but from explicit rules. But more impactful for XML tree builder.

And will particularly help for the XML tree builder.

[jhy]

I'm working on this in the develop branch.