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`

This @ warn should be `$icon-name`, no? Also, if we wanted we could check `variable-exists(icon-list)` and warn on that as well.