merge develop -> master for 0.3 release

fixes for issues in 0.3 release (https://github.com/BladeRunnerJS/brjs/issues?milestone=3&state=open)
highlights:
 - making error for unknown command more obvious
 - first wave of new bundler architecture changes, we aren't using these changes in apps or tests yet though
  - bundler plugins
  - content plugins
  - minifier plugins

this merge doesn't include a release note file for 0.3, that still needs writing