Add AppVeyor CI

[Croydon]

Testing for different VS version is hard/not possible for @lexxmark locally. So let's change this by adding a continuous integration platform and automate this 😄

This generates the following build matrix:

[VS2015, VS2017] x [x64, Win32] x [Release, Debug] + [VS2013 Release win32]

You can see the results here:

https://ci.appveyor.com/project/Croydon/winflexbison/build/1.0.6

AppVeyor is completely free for open source projects. You need to activate AppVeyor before merging this. To do this go to https://ci.appveyor.com login with out GitHub account and enable the winflexbison project.

Happy testing! 😄

[GitMensch]

Thank you, this is really useful, but would be even more useful if we'd integrate the bison/flex testsuites, tracked in #17 and verify that the resulting binaries actually work...
But I assume you could find some sample tests (maybe a little bit more complex bison/flex sources), translate them with the resulting binaries and run the resulting C files through the same C compiler.
@Croydon Can you investigate this?

Additional and likely easier: it would be useful to create artifacts to enable testing from master without the need to actually compile it. @Croydon Would you be so kind?

[Croydon]

I'm sorry, but creating test cases is way out of my scope why I'm contributing here in the first place (I want to package winflexbison, so I can progress in packaging doxygen, so I can package....). If I'm getting lost in every single step on the way then I will totally loose focus 😄
Also I know simply too less about how to use flex and bison.

I would recommend to merge this now to get some form of testing going. Then work on creating tests, if you have tests add them to AppVeyor.

[GitMensch]

Can you still try to add artifacts, please?