Allow some semicolons for one line definitions.

This is a fix for #96 that also has some bearing on #77.

Configurable checking of one line methods. Also allowing one line
classes and modules.

This turned out to be complicated. I hope that goes for the problem, and not only my solution. :-)

The PR only modifies the `Semicolon` cop. New cops for reporting use of one line methods etc have not been added yet.