* Addition of Sass Color Map to Customize Page
Added verbiage to Customization page to demonstrate how to add new colors to the color map.
Using this methodology allows for the dev to use `<i class="fa fa-github has-text-github"></i>` or use `is-github` in a class for a background.
* Change map-merge of $color to only merge once, Sass Complience
- Changed map-merge of $color to import from another map rather than adding each line individually.
- Made it Sass Compliant
Updating based on #1394 comments from @jgthms
