add method `.on()` to assign late events

[phucbm]

Same as viivue/easy-select#15

Sometimes when the popup was initialized using HTML and then we want to assign an event, this method could help to assign events at any time.

Something like this.

const instance = EasyPopup.get('my-popup');
instance.on('change', ()=>{
   // do something
});

[vishalpawarR]

Please can you give more input here and screenshot for better understanding of the problem?

[phucbm]

@vishalpawarR Thanks for reminding me. The description has been updated.