Add ExtraSpacing cop that detects unnecessary whitespace.

Fixes #1434 and #1450
About the algorithm, I think you can simplify it a bit by iterating over the tokens in pairs using `each_cons(2)`. Look at the `SpaceBeforePunctuation` module. This should be very similar: if the tokens are on the same line and token 2's `begin_pos` is more than 1 step away from the `end_pos` of token 1, then it's an offense.