Process YAML frontmatter in .md files

[redoPop]

Allows markdown files to be treated as doc blocks if they open with YAML.

• Example of a markdown file with YAML frontmatter – note that GitHub has special support for rendering YAML frontmatter in markdown.
• Example of HTML output generated from the bem.md source in the previous example. (These files are used for for new tests.)
• Compatibility is maintained for markdown files that don't include YAML; e.g. the index.md fixture. Plain markdown files are treated just as they were before.

Summary of change:

process_files now examines the contents of each .md file to see if its opening lines look like YAML. If so, the file's contents are added to the doc_block_collection.

Resolves #188 since title can be specified in the YAML configuration.

My own use case is producing a CSS code style guide alongside the pattern style guide – i.e., documentation that doesn't really belong in a Sass/scss file. Using .md files for this documentation also makes them readable directly in GitHub, which is a nice bonus.

[jdcantrell]

Sorry for the slow response this is really nice. I'm wondering if the yaml shouldn't have a key specific in the yaml to let hologram know it should parse the yaml and markdown.

[jdcantrell]

The idea being that you might have yaml+markdown for generating something else in your project and you wouldn't want hologram to try and create pages off it.

[redoPop]

Can do! How does this sound:

---
hologram: true
title:    (...etc.)
---

(Markdown body)

• If there is YAML frontmatter in the markdown file and that frontmatter includes hologram: true, then it is processed by Hologram as a doc block.
• Otherwise Hologram falls back to its default handling of markdown files (adding them as pages with an :md key to be handled by the default renderer).

[jdcantrell]

I think it's good, I was trying to think of something else but that's probably the best option for now. Thanks again for the PR.

[redoPop]

Okidoke! I also added an additional fixture for a dummy "jekyll.md" file that doesn't meet the hologram: true test, and added info about the new key to the readme.

An idea: we could avoid the need for the hologram: true flag by looking for a combination of the keys title, name, and one of parent/category/categories in the YAML. That footprint seems fairly unique; if the markdown contains all three perhaps we can be reasonably confident we're looking at a Hologram configuration.