Support Different XML backends

[cfis]

Thanks for this gem!

I found it helpful on [Rice]https://ruby-rice.github.io/4.x/ and now on libxml-ruby.

However, using nokogiri to check libxml_ruby seems weird, and well...wrong. So I updated ruby_memcheck to enable swapping backend xml parsers and included default support for nokogiri and libxml-ruby. I then tested both with nokogiri and libxml-ruby. I left the default require in the gemspec to be nokogiri.

The point of this commit is that it let's me maintain a fork of ruby_memcheck that has only one code change - removing the nokogiri depedency in the gemspec (because if I am testing libxml-ruby I don't need ruby_memcheck to then require libxml-ruby).

I realize this isn't a useful for MR for most other people, but it would make it easy to hook in rexml in the future if someone wanted to use it instead.

Thanks!