Use 'lh' as the vertical rhythm unit

Author: James Kolce

lib/compile.js

@@ -8,7 +8,8 @@ module.exports = (file) => {
 
   return postcss([
     require('postcss-vertical-rhythm')({
-      rootSelector: ':root'
+      rootSelector: ':root',
+      rhythmUnit: 'lh'
     }),
     require('postcss-custom-media'),
     require('postcss-media-minmax'),