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.
You'll need to validate that `$class` is a string here or get a weird error a few lines later