Add icon-address function

Adding this missing icon function that can help make building icon fonts more easy.

For example this will be used to build icon components by looping through an $icon-list.

Status: **Opened for visibility**
Reviewers: @kpeatt @ry5n

## Changes
- Add `_icon-address.scss` file
- Import `icon-address` into `_spline.scss`

Can we strip this out?