Combine two separate list regexps

Adjust group numbers for match data accordingly.
Commands for moving list items up and down
Basic list item promotion and demotion
Smart list item insertion

This patch is based in part on commit
b0cb18393c4b5a81046ace7f116383708104334d
by Matus Goljer.
Remove autoload cookie for `markdown-cleanup-list-numbers'
Extended structure editing keybindings
Less inclusive markdown-regex-list

Don't match newlines, only spaces and tabs.
Fix end detection for empty list items

Check for empty list items of the following form:

 *

Ensure that we don't skip over the whitespace following the marker on
the same line.  Previously, these list items were not being detected
properly by `markdown-cur-list-item-bounds' as a result of this.  In
turn, list item insertion and other functions were broken following
empty list items.
Add newline before new list item when needed.
Untabify

Another whitespace patch, but this seemed like a good time, being just
after a release.
Update docstrings for checkdoc compliance

All functions and variables must have a docstring.
First line should be a full sentence.
All arguments should be referenced as ARG.
Two spaces between sentences.
Functions and variables should be `quoted'.