format only locally changed files

[vvaka]

First of all, Thanks for making spotless
It would be very useful if there is an option to format only files locally modified both in maven , gradle plugins, This allows us to adapt this plugin in existing projects and incrementally format the sources.

This feature exists in another maven-plugin which is not actively maintained, talios/googleformatter-maven-plugin option filterModified

[nedtwigg]

There's a workaround for this in #178.

Totally open to a PR to implement this with a flag, but #178 is the place to propose it.