[WIP] partial_fit for the discrete naive Bayes models

Early PR to review an implementation of `partial_fit` for the discrete naive Bayes models.

TODO:

- [ ] check test coverage and add more tests if necessary
- [ ] check on a non-toy datasets such as the out-of-core Reuters example
- [ ] identify ways to factorize common code
- [ ] update documentation to mention the new method where applicable