Support uploading local image

I have added a feature that support uploading local image to server with Fetch API (only keep smallest)

for example:

var editor = window.pell.init({
  // ...
  // if you want to upload image to server
  // you must config this
  upload: {
    api: 'http://localhost:53595/api/files/img/upload'
  }
})

More detail to visit my repo https://github.com/tatwd/pell if you agree with that, I will send a PR to you.

Thank you!