Switch to use core-layout wrapper

- Use the core layout wrapper
- Indent code-blocks and examples in to the same reading edge as the
  content.
- Add styling for headings and paragraphs used used on the component
  pages, using the core layout resets everything so they need some sizes
  adding back
- Use direct decedent selectors for tag based selectors so they don't
  effect the examples
- Add swap out encoded new lines from fixture data to actual new-lines
Support markdown encoded body documentation

Add support for markdown body blocks to describe a component in more
detail than the summary does. This will let us add some proper
documentation around the usage and expectation of a component.
Merge pull request #6 from alphagov/core-layout

Switch to use core-layout wrapper