Adding a sprintf module.

Implements the standard sprintf functionality in JavaScript. I didn't actually write this, but I didn't see anything equivalent already on the CDN.

Here are the details:
- Origin: https://www.npmjs.org/package/sprintf-js. Using the name _sprintf_ in the directory structure though since there isn't already one. It seems the author only named it _sprintf-js_ because someone else had already claimed _sprintf_ on npmjs.org. The author of the _sprintf_ library though actually points people to _sprintf-js_. Somewhat confusing.
- Version: used the same version number specified on npmjs.org.
- Popularity: 250+ stars on GitHub (https://github.com/alexei/sprintf.js)
- Usefulness: who doesn't love sprintf?