Support uploading local image

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

for example:
```js
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!

Start with the pell.init configuration shown in the issue and review the proposed Fetch API upload flow in the linked example repository. Done means local images can be uploaded through the configured upload.api endpoint while retaining only the smallest image, with the behavior verified in the project.

Written by the indexing model from the issue text.