Add new .icon CSS class to .callout

.icon causes any text with that class to have same colour as callout
border. This is for use colouring FontAwesome glyphs in callouts.

Author: delphidabbler

_sass/_dd-custom-mixins.scss

@@ -9,7 +9,7 @@
 
 @mixin callout-setup($color) {
   border-left-color: $color;
-  h1, h2, h3, h4, h5, h6, var, .var, .kwd {
+  h1, h2, h3, h4, h5, h6, var, .var, .kwd, .icon {
     color: $color;
   }
 }