Allow DivinerPublisher to be specified as a flag

Summary: Allow the DivinerPublisher subclass to be specified via ./bin/divner generate --publisher .... In particular, this allows use of the (mostly broken) DivinerStaticPublisher.

Test Plan: Ran ./bin/diviner generate --publisher DivinerStaticPublisher

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11588