Automatically build and deploy Windows binaries with Travis, the same way it's done for Liblouis

[bertfrees]

The build can be based on Dockerfile.mingw. But instead of building libxml2 and liblouis on the fly, pre-built versions of libxml2 and liblouis can also be used. I use this approach in the liblouisutdml-nar project.

For more inspiration see the Liblouis Travis scripts:

• .travis/before_install/mingw.sh
• .travis/script/mingw.sh
• .travis/after_success/mingw.sh