Added a label option to label ticks in the iNumberLine

Summary: QuickFix to enable more labels on tick marks

Test Plan:
Make an iNumberLine from 3 to 4, tickstep 0.1. Choose
"decimal ticks" as the label style. Check that all tick
marks are labeled

Reviewers: alex

Reviewed By: alex

Differential Revision: http://phabricator.khanacademy.org/D4839