Add an ElementVisitingSuggestorMixin for more advanced codemods

[evanweible-wf]

The AstVisitingSuggestorMixin leverages the AstVisitor interface to help create suggestors, but it only makes the parsed AST available. Some more advanced or complex codemods may require additional information that is only available by resolving the elements via the analyzer. This would require setting up an AnalysisDriver or something similar from the analyzer package to resolve the elements.

Being able to leverage the ElementVisitor pattern would allow consumers to write suggestors that have more information with which to make decisions by being able to obtain things like the source of a type or an element's inheritance tree.

[TimWhiting]

I needed something similar so I submitted a PR would #38 work?