Add support for files that don't have a single root node

The tag-highlighting and the dom-printing features of breeze assume that the document has a single root. While HTML&XML documents should have a single root to be well-structured, this is not always the case - especially with templating engines, that often allow partials - which means that even if the rendered result is well structure, the source files aren't.

This pull request makes tag-highlighting and dom-printing able to deal properly with files that don't have a single root.