blankLine : LexerheadingSym : LexerhorizontalRules : LexerA horizontal rule.
The horizontal rule is reprenseted by three or more consecutive '*', '-', or
'_' characters.
The current implementation allows for the horizontal rule to be directly
following the above block, without a blank line, for example:
```markdown
Hello world!
---
```
However, ideally, there should be a blank line before:
```markdown
Hello
---
```
Space characters are tolerated after the horizontal rule.
imageSym : LexeritalicsSym : LexerboldSym : Lexerlink : LexerhtmlOpenTag : LexerNote: we currently don't allow attributes
To do so, we might need to move this from the lexer into the parser
htmlCloseTag : Lexerpre : LexercodeFence : Lexertext : Lexer