Fix bug in markdown-complete-region/buffer

When searching for horizontal rules, we need to be careful not to match
level-two Setext headings.  Now, pass back over the list of previous
regular expressions to check for matches before completing anything.

Includes a unit test.

Thanks to Gunnar Franke <Gunnar.Franke@gmx.de> for the report.
Fix buffer-wide completion (again)

Closing parens were wrong: the regular expression should not be
added until the outer loop is complete.

Updated unit test to include two incomplete headings.
Filling when a number appears at column zero

Previously, when filling the following paragraph with fill-paragraph,
the "3.14" would not be joined with the previous line, as it was
mistaken for an ordered list item:

    The circumference of a circle divided by its radius is around
    3.14.

Includes a unit test.

This does not, however, address the more general problem:

    The circumference of a circle divided by its radius is around
    3.