3.18.1 Windows builds

[Kai Willadsen <kai willadsen gmail com>]

Hi all,

While there's still a few manual steps involved, there is at least a
Windows build of 3.18.1 up on the main site now.

In related news, the `maint` script now has an `appveyor-build`
command that lets you build an arbitrary tag. This *does* require you
to have your own account set up with a `meld` project under it, but
otherwise should work fairly simply.

I should say that this is all still just getting builds for 3.18
working. The work going on in the other thread is still necessary for
future builds... but at least we have up-to-date stable releases now.

cheers,
Kai