Add local XML ruleset loading

[jayvdb]

Need to remove dependency on https://github.com/EFForg/https-everywhere/tree/master/test/rules/src/https_everywhere_checker , esp. as they want to replace Python with Node.

The local loading code can reside only in tests, but it would be better to support it in the library as XML is a better format for creating/editing custom rulesets.