Feature: Custom nav-link component

[alex-saunders]

• Adds the navItem prop to the <Header> component - a function which takes in a href/link for the given nav item and must return a component. The returned component is then rendered for each <NavLink> (defaulting to an anchor tag)
  • Allows for custom components to be used as links, such as Gatsby's <Link>, e.g:

Aims to fix problems with https://github.com/FormidableLabs/formidable.com/pull/495