nelstrom/vim-markdown-folding — pull request #5 by inkarkat (2012-12-13 13:52:46+00:00)
Title: ENH: Add foldlevel=7 for horizontal rules.

According to John Gruber's spec and the Stack Overflow implementation, horizontal rules are defined by three or more hyphens, asterisks, or underscores (optionally separated by spaces) on a line.

Since horizontal rules are a (weak kind of) structuring mechanism, include them as foldlevel 7, after all possible headings.