Add a Liquid template for Github PR links

[mdlinville]

Generates a Github PR URL from three parameters

Usage:
{% include github-pr.md pr=12345 %}

If you omit the org or repo, they default to docker.
If you omit the pr, it defaults to NULL.
{% include github-pr.md repo=docker.github.io pr=9999 %}

Output:
#12345
#9999

[mdlinville]

Add a unit test to test.md. Go to the /test/ URL on Netlify to test how this works.