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
- [ ] update release notes
- [ ] update documentation
I have a couple of ideas for the documentation.  Let me at least somehow sketch it and then we can return to this particular question.

My idea is exactly that: simplify initial configs (so people do not have to very carefully copy & paste examples from the manual).