Coltrane

- Support Django template tags with the `mistune` markdown renderer.

- Include extra files when building the static site.

- Add `EXTRA_FILE_NAMES` setting to support serving static files like `robots.txt`.

- Add support for rendering markdown with `mistune`. See [MARKDOWN_RENDERED](settings.md) for how to enable. `mistune` will be the default renderer after 0.22.0 because it is 1) faster rendering markdown than `markdown2`, 2) enables new functionality like `abbr`, 3) fixed a bug in the generation of the tables of contents HTML, and 4) has a plugin architecture to add new features.
- Improve table of contents rendering for `mistune`.

- Add `order_by` to `directory_contents` templatetag.
- Fix `TOC` outputting 'None' when it should be `None`.

- Add `to_html` template tag. [37](https://github.com/adamghill/coltrane/pull/37) by [Tobi-De](https://github.com/Tobi-De)
- Breaking change: change `date` to `publish_date` in metadata. [39](https://github.com/adamghill/coltrane/pull/37) by [Tobi-De](https://github.com/Tobi-De)
- Breaking change: change `SITE` setting to `SITE_URL`.
- Automatically add `verbatim` templatetag around code fences.