HELPER HELPERS :P 
Sets the given option to the new-structure options if it's set in
the old/provided. If doesn't exist, it's already set on the new via
defaults.
@private

@param {String} oldProp - Property/option name to be checked on the
old-structure options.
@param {String|Array} oldType - Expected type(s) of `prop` on the
old-structure options.
@param {Object} target - Target object of the new-structure template
options which the value will be set on.
@param {String} [rename] - New property/option name to be set on the
new-structure options.
function setOpt(oldProp, oldType, target, rename) {
        const types = utils.ensureArray(oldType);
        if (oldProp in opts && types.indexOf(typeOf(opts[oldProp])) >= 0) {
            target[rename || oldProp] = opts[oldProp];
        }
    }