function contexts() {
  return this.requestHandler.create('/session/:sessionId/contexts');
}
Returns an object with a value field containing the list of all available contexts

@see http://appium.io/slate/en/v1.1.0/?javascript#automating-hybrid-ios-apps
@see https://github.com/admc/wd/blob/master/lib/commands.js#L279
@type mobile
@for android, ios