Support named captures in UnusedLocalVariables and ShadowingOuterLocalVariable

This fixes #332.

As noted in `unused_local_variables_spec.rb`, MRI 2.0 warns unused named captures only when they are in top level scope. I have no idea why it does so (probably _the inconsistency_) and I think there's no convincing reason to conform to this behavior, so I didn't let `UnusedLocalVariables` mimic MRI in that case.