plugin architecture for tighter SCM integration, etc

you should be able to extend ditz for things like tighter SCM integration.

basic extension points: adding fields and model objects, changing the html and screen output accordingly, adding operations.

use cases: integrate with git. integrate with a mailing list like sup-talk.

currently [unknown issue] gives the ability to add field and model objects, and adding operations can be done by reopening Ditz::Operator. that's a starting point, at least.