A change to format, so that it's evaluated.

Hey, I've been working on implementing spectrum in https://github.com/Textalk/angular-schema-form and while doing that noticed that the format is not being evaluated, so that we can't specify it externally but have to set it once and be done with it. Our use case would have the user specify the output format via a json-form attribute but that's not possible atm. It's a very simple fix, and i feel it should be that way, i do however realise it does break backwards compatibility. Is there a reason it was written this way from the start?

I have updated relevant tests and written a new test for format with a variable.