Added ability to create disabled constraints from code

[dev4dev]

Hello, here is a feature to create disabled constraints along the base layout. This is required sometimes when layout requires changes initiated by disabling/enabling some sets of constraints. Without that there will be layout errors because of constraints conflicts.

view.snp.makeConstraints { make in
    make.edges.equalToSuperview()
    make.height.equalTo(400).disabled()
}

I didn't find the way to achieve that with already existed methods, as a result, added this method.

[kmcginnes]

The way to create inactive constraints is not straightforward, but does exist. You must create the constraint in a prepareConstraints call and store the constraint in your class so it can be enabled or disabled at will

let label = UILabel()
addSubview(label)

var savedConstraint: Constraint?
label.snp.prepareConstraint { make in
  savedConstraint = make.centerY.equalToSuperview().constraint
}

savedConstraint?.deactivate()
savedConstraint?.activate()