feature: add shtfmt formatter for bash/shell scripts #1776

Hello,

I've been using spotless as a maven plugin for a while now, and it's very nice.

The best part is its support for many languages/files.

It would be really nice if spotless would allow using shtfmt (https://github.com/mvdan/sh) to format bash/sh files with a given extension.

Would that be possible?