Tweaked auto-add atmos to be easier to migrate.

* Ported out an abstract ShimWriter form the existing
* Migrated all current shims to new PluginShimWriter
* Added the ability to remove shims which are blank/empty
* Created new generic shim so we can shim non-plugin files
* Use new shim variant to insert Atmos plugin on akka/play projects
* Still waiting on next sbt-atmos release before shiming for play projects.

Review by @havocp
Merge branch 'wip/auto-add-atmos-just-akka' of github.com:jsuereth/sbt-remote-control into wip/auto-add-atmos-just-akka