prev(config, trueVal, falseVal) {
    return this.prevConfig === config ? trueVal : falseVal;
  }
@method prev
@description Return `trueVal` if `config` match previous configuration, in other case - return `falseVal`.
@param {String} config Configuration name.
@param {Any} trueVal Value returned if condition is truthy.
@param {Any} falseVal Value returned if condition is falsy.
@memberof module:modules/app.StateModule