Add support for postcode question

A postcode question acts like a text question with two extra validation points:
- checks the input is a valid postcode
- checks the input is a full postcode (eg "N1" would be invalid)

This could have been implemented by adding validators as a key/value
entry for questions, and implementing a plugin to do that validation.
If more questions different from text only through extra validation are
needed in the future, implementing validators instead of a custom question
type should be considered.
Merge pull request #121 from alphagov/postcode-question

Add support for postcode question