A macro for the Jinja2 template engine.

I am using this for the statically generated site at http://lappland.io.
As an example for how to use it, this is the change for the above site:
https://github.com/hlapp/pelican-bootstrap3/blob/cc-license-mark/templates/includes/footer.html#L14-17

It was motivated by addressing DandyDev/pelican-bootstrap3#41, the issue behind
with was first recorded in hlapp/hlapp.github.io#4.
Adds MIT License file.
Link to Jinja2 template.