Some improvements to the plugins module
Some improvements to the plugins module

- makes events available in a module constant
  - can be used by plugins for inspection
- ensures that registered attributes are actually callable
- does not ignore other results than None that also evaluate
  as False in a condition

jimporter/mkdocs,waylan/mkdocs,jimporter/mkdocs,mkdocs/mkdocs,mkdocs/mkdocs,mkdocs/mkdocs,waylan/mkdocs,jimporter/mkdocs,waylan/mkdocs