Add auto-correct to BlockAlignment cop

I've tested this on a large production codebase and it worked very well.

The auto-correct works on all the spec cases except 'when variables of a mass assignment spans several lines', but it also doesn't do any harm in that case. Some extra spaces may be left at the end of some lines, but the TrailingWhitespace autocorrect takes care of that, so I didn't want to add more complexity to this cop.