Expose a chai global for use in the console

[not-an-aardvark]

This exposes the chai global on every page of the site, allowing people to test things in the dev console.

Being able to easily test a feature while reading its documentation can be very handy, and a few other documentation sites also expose globals for this purpose. (lodash.com exposes a _ global, bluebirdjs.com exposes a Promise global, etc.)

[keithamus]

Great work 😄