Allow RuboCop to run on a Visual Line selection

[radiantshaw]

When you're using version control like git, you only want RuboCop to run on the parts of the file which were changed. Since there are many version control systems, it would be hard to implement this, so an alternative idea is that the users can visually select the text they want RuboCop to run on, and then either hit :RuboCop or \ru. I tried it currently, and it says something along the lines of the command doesn't accept a range.

I can attempt this change and raise a PR if you want. Let me know how you feel about this feature.