Refactoring plugin examples and plugin architecture. 
   - All 'editor hooks' now uniformally accept a single 'editor' param.
   - Added a annotation computer api with automatic life-cycle management.
   - Error handling for annotation computers that throw exceptions
   - Allow annotation computer to return promise if they want to do asynch programming.
   - Add 'priority' mechanism to influence the order of preSave hooks
   - always execute annotation computers after save transforms to avoid loosing the annotations.
   - more consistent terminology: use 'annotations' instead of 'markers' (almost) everywhere.
   - allow disabling plugins by setting scripted.plugin = false in package.json file of plugin.
Merge branch 'dev' of github.com:scripted-editor/scripted into dev