2.2.0

New Features

• Implemented a public API for Task Modifiers,
  providing a way for users to define additional modifiers and behavior in
  application code. (#441) Please consider the TaskProperty
  prototype extension method of implementing task modifiers to be deprecated
  in favor of the registerModifier API.
• Add onError parameter to perform helper to allow UI-triggered tasks to
  specify alternative or null error handlers (#443, fixes #435. Thanks
  @lolmaus for the discussion)

Bugfixes

• Fix inefficent use of run.once (#442, fixes #437. Thanks @stefanpenner for
  the heads up!)