Add transferPropsTo to BlurInput

Summary: Allows the passing of placeholder/className

Test Plan:
1) Open up the input-number widget
2) Change a BlurInput to have a placeholder attribute
3) See if the placeholder is there

Reviewers: alpert

Reviewed By: alpert

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D6905