Validate if the form is pure 0.5.0+1

[andrefedev]

I would like you to add to the FormMixin a method to calculate if the all fields embedded in the inputs field are pure. A good idea to know if the form is pure is by iterating over the inputs field and checking each one of its status.

bool get isPure => inputs.every((e) => e.isPure);

Thanks for the update to 0.5.0+, it was very important to split the statuses of the form field and the general status of the form submission.

[Toavina23]

I have filled a PR for this #87