Match filename to alt text

[samunderwood]

I noticed that filenames are not very SEO friendly, e.g. they're usually a random collection of letters like - fkwgpzwagqc.jpg

It'd be much better if the filename matched the alt text, but replaced and spaces with '-' e.g. an image selected with alt text 'person feeding baby', would have a filename of 'person-feeding-baby.jpg'.

This would help the plugin better follow best-practices given by Google in this article - https://developers.google.com/search/docs/appearance/google-images

[dcooney]

The filename is the actual filename from the API. I like this idea though so happily would review a PR on this :)