User preferences file

Allows a user to have a `.phpspec.yml` in their home folder that sets default preferences.

For instance if they have a local prefs folder with:

```yml
fake: true
formatter: dot
```

And these preferences will be used _unless_ their project has a `phpspec.yml` with contradictory settings
be careful about ``array_merge_recursive``. It will probably not do what you expect. See http://friendsofsymfony.github.io/slides/config_definition.html#6 (and the next slide) for a sample.

Using ``array_replace_recursive`` will probably be better.