Add a selectTokensOfTypes method to the AASTNode class

Summary: Add a selectTokensOfTypes class which allows multiple token types to be extracted from an AASTNode.

Test Plan: This will make more sense in my next diff.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D10121