Appveyor is a CI infrastructure like Travis, but oriented at building on
Windows environment, as opposed to Linux and macOS.
By using Appveyor we can test changes to Epoxy and make sure that
Windows builds won't break.
Additionally, Appveyor allows to deploy build artefacts, which means we
can generate builds for Epoxy releases.
Signed-off-by: Emmanuele Bassi <ebassi@gnome.org>