Structured HR

[jdpage]

Allow horizontal rules to end a section. The syntax is the same as an ATX-style header (see #1) with three or more dashes as the text, e.g. ### ---

A structured rule causes sections to end in the same way as a same-level header, without starting a new section. For example, a level-2 rule ends all open sections, and the rule and subsequent text is in the top-level <article> tag.

A horizontal rule that is too low-level to end any open sections behaves the same way as a regular horizontal rule, and does not affect page structure. It should be rewritten to reflect this.