Support for single item proposition menu

Collapsible menu behavour isn't appropriate for single item menus.
Display the single item in place of the 'menu' link.
This resolves #121 
It would be good to use an absolute (px) size here. `rem` isn't supported in older browsers and [we avoid using it on GOV.UK](https://github.com/alphagov/styleguides/blob/master/css.md#dimensions).