Improve Simple Image Plugin for Editor.js

[nickolasjadams]

The simple image plugin provides a caption for images.
The placeholder use the word caption.

This would be more effective as an alt tag for our project.
So it would be nice to be able to specify it as an alt tag instead.

The quote plugin allows changing the plugin via config. We should fork the original plugin repo, and create a PR with the extra feature included.

Then we can improve our editor package like this in the editor config.

'image' => [
    'class' => 'ArcanelyEditor.SimpleImage',
    'config' => [
        'captionPlaceholder' => 'Alternative Text'
    ]
],