Add parsing of a Doctype tag.
Add parsing of a Doctype tag.

Start using insertionModes and begin the parsing of Doctype tags in our parser.

dushmis/go-html-transform,d0f/go-html-transform,lambdaX/go-html-transform,webfd/go-html-transform,x140cc/go-html-transform,rtfb/go-html-transform,imdario/go-html-transform,wangcheng711/go-html-transform