Implement ThreadSafety/RackMiddlewareInstanceVariable cop

[viralpraxis]

Resolves #15

This [baseline] implementation is pretty straightforward. I applied two heuristic here:

1. we find "rack application" variable (usually named app) and do not report it;
2. we track instance variables initialized in constructor with values which are known to have thread-safe semantics and do not report them.

This cop also supports AllowedIdentifiers concern.

[synth]

Amazing! Thank you!