On the number of spaces in horizontal rules: The spec is fuzzy: "If
you wish, you may use spaces between the hyphens or asterisks."
Markdown.pl 1.0.1's hr regexes limit the number of spaces between the
hr chars to one or two. We'll reproduce that limit here.

# no spaces

---

***

___

# one space

- - -

* * *

_ _ _

# one space with some leading space

  - - -

 * * *

   _ _ _

# two spaces

-  -  -

 *  *  *

  _  _  _

# three spaces (these shouldn't be hr)

-   -   -

 *   *   *

  _   _   _

# one or two spaces

- -  -

 *  * *

  _ _  _

# longer

- -  - -  - -  - -

 *  * * *  * * *  *

  _  _ _  ___  _ _  _



