Add the ability to dump theta values into CSV format

Summary: When I was looking into improving Challenge Cards with different recommendation thresholds per-exercise, I wanted to analyze the average magnitudes of the theta values themselves. So, I added this function to dump_knowledge_model_metadata.py so I could see them, and others in the future can, too!

Test Plan: `python src/dump_knowledge_model_metadata.py -t ~/dev/webapp/exercises/new_model3.pkl  | head`

Reviewers: jace

Subscribers: tom, tony, eliana

Differential Revision: http://phabricator.khanacademy.org/D7454