function documentMetadata(doc, key, newValue) {
  if (newValue) {
    sketch.Settings.setDocumentSettingForKey(doc, key, newValue)
  }

  return sketch.Settings.documentSettingForKey(doc, key)
}
Set or get metadata stored in a specific document.

@param {MSDocument} doc
@param {String} key
@param {String} newValue
@return {String}