Support marking unused parameters by prefixing them with _

The mbj/mutant gem also uses this convention, and
there simply are situations where you need to have
the variable named, but you know that it won't be
used.

This commit contains all commits done recently in
troessner/reek#141 squashed into one.
Merge pull request #142 from snusnu/pimp-unused-parameters

Support marking unused parameters by prefixing them with _