Add support for type parameter being GitHub repository file name

[bobbingwide]

I've been looking at a number of GitHub repositories which are for Genesis child themes and decided I wanted an easy way to display links to the repositories using their screenshots.
It makes sense to extend the [github] shortcode to display the image when the type parameter is a file name which is an image (e.g. .jpg, .png)

The image displayed would have an URL like this
https://raw.githubusercontent.com/bobbingwide/oik-bob-bing-wide/master/assets/oik-bob-bing-wide-banner-772x250.jpg

Shortcode example:
[github bobbingwide oik-bob-bing-wide assets/oik-bob-bing-wide-banner-772x250.jpg]

While we're at it, we may as well support other files.
Shortcode example:
[github bobbingwide oik-bob-bing-wide shortcodes/oik-github.php]

https://github.com/bobbingwide/oik-bob-bing-wide/blob/master/shortcodes/oik-github.php

We won't default the file name.
So a link to a theme by its screenshot.png file would be
[github bobbingwide genesis-oik screenshot.png]