Add admonition icons and ability to use icon macro to the playground

The ability to insert icons into their content and use icons for the admonitions is an important (and fun) feature for a lot of people. Currently the playground uses the text labels for the admonitions and the icon macro doesn't work.

Could we wire up Font Awesome to the playground, turn on the font icons for the admonitions, and have the font icons work with the icon macro?