Plugins

Docs is still missing.  Basic infrastructure is in place.

All kind of comments are more than welcome :)

- [x] basic infrastructure
- [ ] adopt it for built in stuff
  - [x] www
  - [ ] slaves
  - [x] change sources
  - [x] schedulers
  - [ ] steps (seems the most "interesting")
- [x] provide unit tests
- [x] update release notes
- [ ] update documentation
In that case, it's probably better to define a custom exception for this module, and then catch that exception at the appropriate place in the config-loading process.

This module isn't config-specific.  For example, I can see `info` below being called at runtime by the web status.