allow parameters in extension

This is a **BC Break**.

You must change `phpspec.yml` accordingly: 

    # before
    extensions:
        - Some\Extension

    # after:
    extensions:
        Some\Extension: ~
        # or, and that's why
        Some\Other\Extension:
            option1: ~


I started to spec this until I realised this class is not speccable with current impl.

We should extract this extension loading mechanism to its own class.
Sorry, you're right. That's tripped me up before. My bad!

Yes I don't think this is an acceptable break. I'd go with a extension of ExtensionInterface that has `setParameters(array $params)` or similar