Add additional Whitehall format types

This extends the list of formats from Whitehall, to include publication,
speech and news story sub-types, so that we can register all the
Whitehall editions with Panopticon.
Merge pull request #119 from alphagov/add-whitehall-format-types

Add additional Whitehall format types

https://www.pivotaltracker.com/story/show/65798220