Added a placeholder to drop down widget.

Summary:
Added a placeholder value to the drop down widget. Allows content
creators to give users some idea of the options available in the drop down.

Test Plan:
- Run Perseus
- [Here](http://localhost:9000/test.html#content=%7B%22question%22%3A%7B%22content%22%3A%22%5B%5B%E2%98%83%20dropdown%201%5D%5D%5Cn%22%2C%22images%22%3A%7B%7D%2C%22widgets%22%3A%7B%22dropdown%201%22%3A%7B%22type%22%3A%22dropdown%22%2C%22graded%22%3Atrue%2C%22options%22%3A%7B%22choices%22%3A%5B%7B%22content%22%3A%22One%22%2C%22correct%22%3Afalse%7D%2C%7B%22content%22%3A%22Two%22%2C%22correct%22%3Atrue%7D%5D%2C%22placeholder%22%3A%22One%3F%20Two%3F%22%7D%7D%7D%7D%2C%22answerArea%22%3A%7B%22type%22%3A%22multiple%22%2C%22options%22%3A%7B%22content%22%3A%22%22%2C%22images%22%3A%7B%7D%2C%22widgets%22%3A%7B%7D%7D%2C%22calculator%22%3Afalse%7D%2C%22hints%22%3A%5B%5D%7D) is an example case

Reviewers: jack

Reviewed By: jack

Differential Revision: http://phabricator.khanacademy.org/D9229