Mark generated code as generated code

[ahordijk]

To prevent code analysis warnings and errors.
Please add [System.CodeDom.Compiler.GeneratedCode(null, null)] to all generated code, with appropriated tool and version number.

[SteveDunn]

I added something similar to Vogen recently: SteveDunn/Vogen#126

[andrewlock]

Done as part of the refactor in #117. Note that this marks the whole type as generated, even though it might not all be (because you might manually add additional implementation). It's probably ok though IMO, hence implementing it 😄