SBT support

[nedtwigg]

@moznion has made a spotless plugin for the sbt buildsystem. I bumped into it the other day, and I began a conversation with the author about the option of upstreaming it moznion/sbt-spotless#22

We should definitely let our users know that it exists by linking to it in the root README. I think it would also be good to upstream it into the spotless repo. The advantage of that is that when we add features / fix bugs in various steps, it would be easier for sbt users to know which versions to use. For spotless users that don't use sbt, it helps to have more eyes on the various steps - you never know who might fix a bug or bring a feature.

The downside of having more build steps is that our build gets bigger and slower. Right now, we have the ability to exclude the maven plugin, we could always change it to where all the plugins are excluded, and you have to manually set SPOTLESS_INCLUDE_GRADLE or SPOTLESS_INCLUDE_SBT, so that developers working on the build system they care about don't get bogged down by the larger scope.

spotless/settings.gradle

Lines 47 to 49 in 74d964d

	if (System.getenv('SPOTLESS_EXCLUDE_MAVEN') != 'true' && System.getenv('JITPACK') != 'true') {
	include 'plugin-maven' // maven-specific glue code
	}

The main challenge to upstreaming @moznion's work is just

• a PR with a folder plugin-sbt
• add the folder to settings.gradle
• get it building via gradle, so that our publishing works https://docs.gradle.org/current/userguide/scala_plugin.html