Adds the_author_image_url() to return only the raw URL

This is my first attempt at a pull request, so I hope you'll bear with me. 

I've used the Author Image plugin for a number of things and love it, but in a new project I needed the ability to pull the image and format it my own way, which I couldn't find in the docs or looking at the code, so I added a simple function that returns the URL of the image once it's pulled instead of returning formatted code. The addon has been updated since then, and I'd love to continue updating it in the future, but don't want to lose this functionality so I'm suggesting it as an addition to the main plugin. My PHP and WP knowledge is intermediate, so there may be a more elegant way to do this and I'm open to suggestions for alternatives.

Thanks for looking!