Add a [github] shortcode to link to GitHub

[bobbingwide]

Now that I'm making better use of git, raising GitHub Issues, and tagging releases, etc..
I want a shortcode to be able to quickly reference GitHub repositories, issues, releases and stuff.

Proposed solution

[github] shortcode.

This first version will simply be used to create links to GitHub.

The shortcode will accept positional parameters representing:

• owner
• repository
• content type
• identifier

so that I can type something like
[github bobbingwide oik-bob-bing-wide issues 1]
and end up with a link to this issue.
#1

Initial limitations

It'll be very basic. It won't use the GitHub API to extract information from GitHub. It'll just make links a bit friendlier than typing the whole URL. The 'text' part of the link will be a bit more pleasing to the eye.