Allow default letterSpacing in fontSize config

Extend the configuration syntax to allow specifying letterSpacing
alongside lineHeight in the fontSize config.

The syntax looks like
```
module.exports = {
  theme: {
    // ...
    fontSize: {
      sm: ['14px', { lineHeight: '20px', letterSpacing: '0.01em' }],
      // ...
    }
  }
}
```

Also retains backwards compatibility with array syntax of 1.3

Author: valtism

src/plugins/fontSize.js

@@ -4,7 +4,11 @@ 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, options] = Array.isArray(value) ? value : [value]
+        // Tailwind 1.3+ syntax allowed line height to be specified in the array like
+        // ['16px', '24px'], so we can get it from there as well as from object syntax
+        const lineHeight = options instanceof Object ? options.lineHeight : options
+        const letterSpacing = options && options.letterSpacing
 
         return [
           `.${e(`text-${modifier}`)}`,
@@ -15,6 +19,11 @@ export default function() {
               : {
                   'line-height': lineHeight,
                 }),
+            ...(letterSpacing === undefined
+              ? {}
+              : {
+                  'letter-spacing': letterSpacing,
+                }),
           },
         ]
       })