Create ButtonGroup component

Summary: A ButtonGroup component that accepts (by default) 0 or 1 buttons selected, and (by choice), exactly 1 button. Pretty well-documented, so you can just read the comments.

Test Plan: Tested in number line given new changes.

Reviewers: jack

Reviewed By: jack

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D6899