Refactor removing style tags

This uses xPath to find the style tags and either remove the style tag completely, or replace them with just the media queries, based on the excludeMediaQueries flag.
This follows the logic that if the mediaqueries are excluded from inlining, they should not be stripped from the document, because they aren't processed.
If for some reason they are processed (don't know the use case) they can just be removed.

This uses xPath instead of the earlier regex, because this seems easier/more reliable. I did think about changing the finding of styles to also use xPath, but according to the docs/comments the order in which the results are returned isn't 100% reliable.

Replaces #54
Shouldn't we add `use DOMXPath` at the start of the file, and then ref this without the slash - just a minor cs point. :)