Handle =for vim formatting command

The colons in the command would break =for in Pod6, so change it to a
=comment.
Fix =item's, resulting improvements

Use for ^+@pars instead of @pars, so we can check previous/next
paragraphs for fancier stuff. The final join("\n") had to be replaced
with a join(""), and newlines added to strings as they're pushed to
@outpars, to better control newline placement.

All this results in the first line of =item statements appearing on the
same level as the bullet.