Automatically run GC/finalization after each task execution.

Fixes #1223.

* Add a new key to disable forcing the garbage collector to run
  after each task-executioin
* Add a new flag to disable forcing the garbage collector to run
  after each task-exeuction
* Add a hook into EvalauteTask to run System.gc/System.runFinalization
  after each task execution

Review by @eed3si9n
Merge pull request #1276 from sbt/wip/run-finalizer-after-tasks

Automatically run GC/finalization after each task execution.