Add a "--clean" flag to "bin/translatewiki export" to drop project caches

Summary: Ref T5267. When the cache format changes (as it did when I added types recently) make it easier to get a clean rebuild.

Test Plan: Ran bin/translatewiki export --clean, got clean exports.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T5267

Differential Revision: https://secure.phabricator.com/D16825