Rename the "contrib" directory to "completion".

The original "contrib" name is lousy. Everything in notmuch has been
contributed, and we are integrating as much of it as possible, (rather
than making users grub through contrib looking for useful pieces to
install).

Meanwhile, the only things we have in contrib are command-line
completion scripts, so "completion" makes more sense as a name, (and
helps make "./configure" slightly less ambiguous).