Confine link_label to inside of square brackets
Add relevance links

This is so markdown isn't guessed as asciidoc.
Use variants for markdown
Add header support for 1-6
Include a single tab to match as code class
Include three `*` in horizontal_rule class
Add reference linking in markdown
Add test for reference links for markdown
Add link_reference to solarized themes
Add link_reference to docs
Group begin and end together
Search for links that are referential
Add reference_link class in list of contains
Merge branch 'markdown-improvements'