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.
@GrahamCampbell well, the idea of @ciaranmcnulty would work too: http://3v4l.org/VBgGP

Which means: keep the extension interface as is, and add a new, independant one, that handles params.

Indeed, we would have to check for instance type, and handle the 2 cases.
Don't know if it worth it (we could BC break and upgrade major version), but it's definitly better than `setParameterers` because it would allow more flexible extensions, and it smells way too procedural to me.

Anyway, if everybody is ok with setParameters, I'll implement it with pleasure.
HO, it just came to my mind that adding a method is as much of a BC break than adding a parameter :)
which means if we want to maintain compat, we'll have to introduce 2 interface and check types in any case!

That's why I'd prefer solution 1 (http://3v4l.org/VBgGP)