Govspeak for view templates.

- Add govspeak gem
- Register govspeak as a handler for view templates ending in `.md`
This is overly complicated, move `.find` to `#find` and drop the `instance_eval` stuff. You should probably also pass the root path in on the constructor as its a hardcoded dependancy which is hidden right now in the `#path` method.

For maximum obviousness you could do something like this:

```ruby
class ArticleRepository
  DEFAULT_ARTICLE_DIRECTORY = Rails.root.join('content')

  def initialize(article_directory = DEFAULT_ARTICLE_DIRECTORY)
    @article_directory = Pathname.new(article_directory)
  end

 ...

private
  attr_reader :article_directory

  def article_path(id)
    article_directory.join("#{id.tr('-', '_')}.md")
  end
end
```

As an added bonus this will be easier to test, and extracting the govspeak stuff will easier in future (if needed).