Added footer-color option (#2480)

* Added foreground (e.g., text) color to the footer

While setting up my own site, I was surprised to find that this wasn't an option!

* Added @if statement for footer color

* Added the option for footer-color

* Corrected indentation of lines for footer-color

sass/layout/footer.sass

@@ -1,6 +1,9 @@
 $footer-background-color: $white-bis !default
+$footer-color: false !default
 $footer-padding: 3rem 1.5rem 6rem !default
 
 .footer
   background-color: $footer-background-color
   padding: $footer-padding
+  @if $footer-color
+     color: $footer-color