Add support for tabs as indent

[JakeWharton]

The whole tabs vs. spaces debate is exhausting. I use tabs, and lots of other people use tabs. There's really strong arguments for preferring tabs. It would be nice if this tool supported tabs, too.

Since there is already a mechanism in place to switch between 2 and 4 spaces, adding tab support should not be wildly challenging. If you are open to supporting tabs, I might even be able to contribute it (I added tab support to ktlint).

Just think: if you only supported tabs, users could pick 2 or 4 character rendering (or any other value) and you wouldn't have needed an option at all!