Don't stem "words" with a "." or "_" in them

Summary: Fixes T12106. It is correct for us to stem "oddly-proportioned", but not "oddlyproportioned.com" or "maniphest.levelup" or "ad_hoc_token_thing".

Test Plan:

• Added a bunch of test coverage.
• Created a task named "apples.bananas ducks_quacks". Found it by searching for "apples.bananas" and "ducks_quacks".

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12106

Differential Revision: https://secure.phabricator.com/D17330