function getOriginalLandingPage(opts = {}) {
  const config = Object.assign({}, CONFIG, opts)
  const key = config.originalLandingPageKey
  const storageConfig = { storage: config.storage }
  // 1. try first source browser storage
  const originalLandingPage = getItem(key, storageConfig)
  if (originalLandingPage) {
    return originalLandingPage
  }
  const url = (isBrowser) ? window.location.href : ''
  setItem(key, url, storageConfig)
  return url
}
Get the original landing page of a visitor
@param  {Object} opts - plugin
@parama {String} opts.originalLandingPageKey - Storage key
@parama {String} opts.storage - Storage type
@return {String} original landing page uri