Add auto-correct to BlockAlignment cop

This is a modified version of #1002. The original PR went beyond the scope of this one cop, so this no longer includes the extra functionality. (That will be taken care of by a new cop, which I'll open a PR for in a bit.)

Note that the class disables the ClassLength cop, because it is too long with the extra functionality. It didn't make sense to me to extract the autocorrect into a module, since it isn't shared anywhere, just to satisfy the length limit. Let me know if there's some other approach that you think makes more sense here.