Set alt tag for img if there is figure caption

#1

Suggest to add alt tag when adding an image through the admin interface (not markdown) with a figure caption. It will be useful for seo.

I wrote a line of code, test it and it works.

file Ghost/core/server/lib/mobiledoc/cards/image.js line 39

        if (!payload.alt) {
            img.setAttribute('alt', payload.caption);
        }

I cant commit in repo, so maybe somebody who can will take this.

Alt text for images pasted into a blog post