Feature: break constraint validation option

[lilianchiassai]

In the current version, all constraint directives are run when validating an input. This is usually a good thing, but we might want, sometimes, to stop the validation if a specific constraint is not met.

For example: let's define a new constraint @ComplexConstraint.

type Mutation {
  createObject(input: [CreateObjectInput] @NotEmpty @ComplexConstraint): Object
}

In this case, it would be great if the @ComplexConstraint would not be run if the input is empty.

Another use case is a @lookup constraint. When run, it checks if an ID exists in the database.

type Mutation {
  updateObject(id:ID @Lookup, input: UpdateObjectInput @ComplexConstraint): Object
}

Here, the mutation should try to validate the input only and only if the object to edit exists in the database.

[setchy]

Resolved by #29 #37