function createAuthIsReady(store, config) {
  return typeof config.authIsReady === 'function'
    ? config.authIsReady(store, config)
    : authIsReady(store, config.firebaseStateName)
}
Function that creates and authIsReady promise
@param {object} store - The Redux store on which we want to detect if
Firebase auth is ready.
@param {object} config - Config options for authIsReady
@param {string} config.authIsReady - Config options for authIsReady
@param {string} config.firebaseStateName - Config options for authIsReady
@returns {Promise} Resolves when Firebase auth is ready in the store.