R1:2133e610480e

Added placeholder support to custom text fields

Summary: placeholder text is pretty useful.

Test Plan: placeholder text is pretty useful. also fully supports not breaking everything.

Reviewers: chad, #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9223