Alternate Opt-in attributes

[Andrew-Hanlon]

This is a fantastic tool, but I think one of the main barriers to wider adoption is the (perhaps unwarranted) fear of applying code re-writing across an entire codebase.

I think there would be a much larger/faster adoption of the tool if there was an additional set of attribute that allowed per-class or member opt-in vs. opt-out.

This could come in the form of an assembly level opt-out attribute (like the NoLinqRewriteAttribute at the assembly level), and an opt-in attribute at the class/member level (like a 'ApplyLinqRewriteAttribute').

I would be happy to create an initial PR if this was something that sounds acceptable.