Provide a default tracking with Font Size config

We can already configure line-height with font sizes, but this allows
us to also specify the default tracking for these sizes too.

This can be overwritten with tracking utilities.

Author: valtism

src/plugins/fontSize.js

@@ -4,7 +4,7 @@ export default function() {
   return function({ addUtilities, e, theme, variants }) {
     const utilities = _.fromPairs(
       _.map(theme('fontSize'), (value, modifier) => {
-        const [fontSize, lineHeight] = Array.isArray(value) ? value : [value]
+        const [fontSize, lineHeight, letterSpacing] = Array.isArray(value) ? value : [value]
 
         return [
           `.${e(`text-${modifier}`)}`,
@@ -15,6 +15,11 @@ export default function() {
               : {
                   'line-height': lineHeight,
                 }),
+            ...(letterSpacing === undefined
+              ? {}
+              : {
+                  'letter-spacing': letterSpacing,
+                }),
           },
         ]
       })