Add customisation variables

sass/elements/icon.sass

@@ -1,24 +1,29 @@
+$icon-dimensions: 1.5rem !default
+$icon-dimensions-small: 1rem !default
+$icon-dimensions-medium: 2rem !default
+$icon-dimensions-large: 3rem !default
+
 .icon
   align-items: center
   display: inline-flex
   justify-content: center
-  height: 1.5rem
-  width: 1.5rem
+  height: $icon-dimensions
+  width: $icon-dimensions
   .fa
     font-size: 21px
   // Sizes
   &.is-small
-    height: 1rem
-    width: 1rem
+    height: $icon-dimensions-small
+    width: $icon-dimensions-small
     .fa
       font-size: 14px
   &.is-medium
-    height: 2rem
-    width: 2rem
+    height: $icon-dimensions-medium
+    width: $icon-dimensions-medium
     .fa
       font-size: 28px
   &.is-large
-    height: 3rem
-    width: 3rem
+    height: $icon-dimensions-large
+    width: $icon-dimensions-large
     .fa
       font-size: 42px