Add a template rendering scope method in hologram_markdown_renderer

- This allows for the ability to define a haml rendering scope when
  using the 'custom_markdown_renderer' option in hologram_config.yml.
  We no longer need to rewrite the entire render_html method when
  defining a custom_markdown_renderer.
Merge pull request #63 from jfmyers9/master

Add a template rendering scope method in hologram_markdown_renderer