Govspeak Component

Create a component for govspeak. All this will do is output the content
passed to it. By being a component it means that the styling can be
managed in a consistent place, so changes can be rolled out to all of
our text blocks in one go.

The Sass for this has mostly been lifted straight out of whitehall with
some minor alterations (things like using standard typography mixins
rather than whitehall specific ones).

Also updates the docs to be YAML rather than JSON so that we can
use multiline strings in fixtures.

I would appreciate feedback on naming and the api this component 
uses. The Sass can be iterated later if I have missed use-cases as.