Plugins and Extensions

[hartym]

Plugins and Extensions should have a stabilized API, and somehow a requirement list to be considered "official" (and maybe automatically include documentation from official plugins in the main documentation site).

This probably also means that all "contrib" packages should move in their own package, which would solve a lot of testing trouble with extensions that needs dependencies (optional for bonobo, but required for extension).

It also means that there should be good documentation on writing plugins, writing extensions, the scope of each, etc.

A bit of info is available in #257