Squashed 'sbt-extras/' content from commit b170cdd

git-subtree-dir: sbt-extras
git-subtree-split: b170cddf3474c9690bd5150a8d09a2f292fb84c6
Merge commit '6877c83ac368115f5a04f5e5b7d0d4d1652053c5' as 'sbt-extras'
Copy sbt-extras/sbt to root.