Autogenerate .travis.yml and .appveyor.yml?

[bassosimone]

Currently, the documentation describes how these files should look like. It would be better to generate them when mkbuild is run. This would allow us to automatically pick up advancements in the build technique. For example, github.com/measurement-kit/mkuuid4 is currently the only repository that also uses MingGW for building on Windows. Rather than having to manually change the .appveyor.yml of each repository, I'd have mkbuild regenerate it for me. Does it make sense?