Add `icons` folder and helper/shortcode

[Paul-Hebert]

Once #34 is done it would be awesome to scaffold out support for an icons folder. You could then drop SVGs into this folder and a few things could happen:

1. They would display on a new Icons page in the pattern library
2. They could be easily embedded as a short code.

[Paul-Hebert]

It seems like this would still fit inside a boilerplate even though we'd technically be adding a pattern.