added interface matcher

[marcgreenstock]

Thanks for making this, I was just about to start my own until I discovered you have already done so.

This PR adds an interface matcher. Hope you like it :)

[khamusa]

Thank you @marcgreenstock! This is definetely missing!

I'm not sure about the naming. I feel it doesn't read very well expect(...).to interface. I'd rather use implement which is the term used on the overall graphql docs. What do you think of expect(...).to implement(interface1, interface 2...) ?

[marcgreenstock]

hey @khamusa you're absolutely right. interfaces is deprecated anyway. I will change it now, just a sec.