Add simple text field question.

A precursor to [this story](https://www.agileplannerapp.com/boards/105200/cards/5222), adds a simple text field question to smart-answers using [Smartdown](https://github.com/alphagov/smartdown).

The `label.for`, `input.id` and `input.name` will need to be unique, as this question could be on a page with others.

There's a good example for this: https://github.com/alphagov/smart-answers/blob/add_text_field_question/app/views/smart_answers/_multiple_choice_question.html.erb#L6