[WIP] Refactored Config and ConfigInterface

**Note**: This is work in progress. Do not merge yet.

* [x] Refactor the Config class and its interface to use `OptionsResolver`
* [x] Fix tests
* [ ] Add test for `allowedValues` and `normalizers` in the config class.
* [ ] Add doc blocks for the new methods. Sorry, no IDE on my side.

Well. This is a first shot to tackle issue https://github.com/wsdl2phpgenerator/wsdl2phpgenerator/issues/94. This definitely needs review, because after changing the config class and the interface, I just fixed the tests to work again and I must admit that some parts of the code are still a black box to me.

I think the best would be if someone with write permission could create a version branch `v3` where I can rebase this PR to. So this is just to get some reviews and further input on this topic. :)