R1:8f70d891fa8d

AphrontFormSelectControl: Add <optgroup> to <select>

Summary:
Add <optgroup> style selects, if the array of options is actually an
array-of-arrays.

Test Plan: Made one, it looked OK.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran

Differential Revision: https://secure.phabricator.com/D2177