Add no_code_generation and stop_on_failure to config

fixes issue #352

This adds the following options to `phpspec.yml'

```yaml
no_code_generation: true
stop_on_failure: true
```
I've been having a look at this and I could easily wrap it up on a simple class and pass it in. 

However what I'd like to do it take some time to improve the whole approach to config and build a nice modular and extensible config system which would also remove some of the responsibility from the `Application` class. The problem however would be that it would break any extensions which currently make use of config settings in the yaml and they would need updating. Is this something worth considering or is it a bad idea and I should just stick to the first option?

Also if it's not a good idea would it still be work submitting a PR for it with a view to adding it to a major release update in the future?