Rewrite StructuredText::asHtmlText

This rewrite uses DOMDocument rather than manually editing the HTML
string.

We drill down to the right text node based on the required offset of
the span, split it in the necessary places and wrap it in the new
node. We then don't need to worry about the new tags causing offset
changes, since the DOM model doesn't count the tags as text.

It should be much more reliable; it's certainly much easier to read
and understand, in my opinion.

This patch also closes a couple of potential issues and one definite
issue: we skip over spans which are invalid (either improperly nested
or which end before they start).

There are commented out `throw new \Exception` lines where warnings
could be emitted. I don't want to throw exceptions because I still
want some rendered HTML.
Merge branch 'bugfix/handle-invalid-span-positions' into patched

Conflicts:
	src/Prismic/Fragment/StructuredText.php