Wish: support XPath variables

[garfieldnate]

It would useful if XML::Twig could handle XPath variables using the support contained in XML::XPath.

I'll be attaching a pull request for very basic support of this (including a test, and some todo notes :) ). I don't know if it's what you'd want the API to be, but it is a basic proof-of-concept. I don't really know a lot about the Twig codebase or I would do more. Also, sorry in advance that my editor trims trailing whitespace (there are a lot of extra deletions in the commit).

See also SO discussion.