Govspeak for view templates.

- Add govspeak gem
- Register govspeak as a handler for view templates ending in `.md`
Add site prism.
Return govspeak as an HTML quoted string.
First feature for content designers writing articles.
Show article with hyphens in the URL.
Add article model with title and govspeak content.

Instead of ActionView::Template::Handlers that render markdown content.

Enables a content editor to:

* Add an article written in govspeak to `views/articles` with the `.md` extension, e.g. `example_article.md`

* The article should appear at `http://localhost/articles/example-article` with the article layout and the page title being the first level one header

@andrewgarner @gjvis :eyes:
Rewrite ternary statement.
Remove obsolete example article.
Add article repository and content directory.
Find the article header and set the title instance variable once.
Change article attribute accessor to reader.
Inline method that returns article content as HTML.
Cache the govspeak document.
Refactor article repository to use instance methods.
Remove article equality operator only used in tests.
Update addressable gem.
Specify govspeak gem version.
Add travis configuration.