build,windows: build and test add-ons on CI

[bnoordhuis]

Continuation of #2428. Building and testing add-ons has been implemented on Unices but not on Windows.

This probably needs to be added to vcbuild.bat although I'm unsure what the best way to do that is. Call out to a python script that builds the add-ons? Normal add-ons in test/addons can be built from a master binding.gyp but that won't work for add-ons scraped from the documentation.

/cc @orangemocha @nodejs/build @nodejs/platform-windows

[Trott]

/cc @nodejs/testing