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!

[hick209]

ktfmt was written based on top of Google Java Format, with the same philosophy on keeping it very simple and without customizations, at least for the out of the box experience.

While I don't foresee folks internally agreeing on adding tab support (I can kickstart a discussion around this), we do allow people to create their own jar adapting things as they see necessary from the project.

In other words you could expose those tools at the coding level and use that to get a custom jar for your use case. I would not oppose this at all.

[JakeWharton]

Took a quick look, and this seems blocked on GJF whose indent representation and output is solely around number of spaces.

Filed google/google-java-format#1320