Add a publish cache for the Diviner static publisher

Summary: Keep track of what we've written to disk, and regenerate only new documents.

Test Plan: Changed a small number of files, saw that number of files get regenerated. Ran with "--clean" and saw everything regenerate.

Reviewers: btrahan, vrana, chad

Reviewed By: chad

CC: aran

Maniphest Tasks: T988

Differential Revision: https://secure.phabricator.com/D4897