Add custom Cows and Figlet directories to .gitignore

Summary: These directories didn't get added to .gitignore, but should be.

Test Plan:

• Touched a dummy file in each directory.
• Ran git status.
• Didn't see the file show up.

Reviewers: chad, cspeckmim

Reviewed By: chad

Differential Revision: https://secure.phabricator.com/D14249