Add a script to easily view new submitted plugins

Summary: That is, submitted plugins that we don't already know about and are non-empty.

Test Plan:
- imported the `submitted_plugins` table from prod
- ran `PYTHONPATH=. python tools/clean_submitted_plugins.py`
- saw the output looked reasonable

Reviewers: xymostech, spicyj

Reviewed By: spicyj

Differential Revision: http://phabricator.benalpert.com/D213