Add auto-lists

This feature lets Vim automatically add a new list item on <CR> (or end
a list if the list item is blank). In the case of ordered lists, the
list item number is incremented.

This is a prototype, an idea. It seems to work normally, but one feature is missing, because I don't know how to program an efficient implementation:

If a list item spans over multiple lines, a list item won't be added.

Also, the only list item character supported for unordered lists is the dash (`-`). I will add the support for asterisks (`*`) ASAP.