#207 adder method for collections on target

For review, it is not yet completely finished. It contains TODO's in the `MapperCreationProcessor.java` and `Type.java`. It also lacks more unit test (placeholders are in place).
> But as you say a project will either follow that style or it won't, so I think it makes sense to be able to enable it on a more global level then specific properties, i.e. per an attribute in @Mapper or even as processor option.

That's what I meant. We have the `@MapperConfig` to override on a project level. So lets use that in combination with `@Mapper`