Support Promise API

Breaking change

• Support the Promise API

• Deprecated prefix option and callback option which has been replaced by Promise API

- new Jsonp({
+jsonp({
  url: 'http://localhost',
  // global function named `${jsonpCallback}` will be invoked when JSONP response
  jsonpCallback: 'jsonp',
  timeout: 5000,
  // eg. ?customCallbackParams=...
  callbackParams: 'customCallbackParams',
  urlParams: {
    // eg. ?key0=0&key1=1...
    key0: 0,
    key1: 1
  }
- prefix: 'prefixOfCallback'
- callback: ()=> {...}
})
+ .then(res => res)