Add a linter rule for the use of parse_str

Summary: The use of the parse_str method (when called without sepcifying a second parameter) hinders static analysis. Specifically, the parse_str('...') behaves similarly to extract.

Test Plan: Added unit tests.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D13857