Allow root plugins to be disabled.

Fixes #1455

* Add a mechanism to detect if a plugin clause includes/excludes
  a particular plugin.
* Add a filter for our "enabling" clauses so that user-excluded
  root plugins do not show up.
* Add a test to ensure this succeed.
Added notes so we have a merge conflict for @eed3si9n to deal with.
So apparently scalariform was unable to parse these files.
Remove printlns.
Merge pull request #1516 from sbt/wip/fix-1455

Allow root plugins to be disabled.