This commit allows a help message to be printed when a `-help` flag is
passed to the editor, which will subsequently quit after a summary and a
description of all the flags available have been displayed.
The GNU convention (passing a single `--help` argument to the program)
is also supported, although undocumented.
The man page also now documents the `+:` argument, although unrelated to
the original changeset.
16068321c1
