Style contacts in markdown blocks

- Add some styling
- Let you specify the heading tag for contact blocks
- Use a paragraph for the address as it didn't have a semantic wrapper

https://www.pivotaltracker.com/story/show/48480651
Merge pull request #614 from alphagov/govspeak-contacts

Style contacts in markdown blocks

https://www.pivotaltracker.com/story/show/48480651