Add EmptyLinesAroundBlockBody cop

There were a few existing cops for preventing/enforcing newlines
around the body of classes, methods, and modules. There wasn't one
for blocks (especially relevant for rspec!) so this adds one.
Fix EmptyLinesAroundBlockBody offenses
Merge pull request #1463 from jcarbo/empty-lines-around-block-body

Add EmptyLinesAroundBlockBody cop