Add Rollup plugins

Add Rollup plugins, includes commonjs plugins since GOV.UK Frontend is currently shipped as UMD.

See alphagov/govuk-frontend#711 for more details.

lib/metalsmith.js

@@ -11,6 +11,10 @@ const permalinks = require('metalsmith-permalinks')          // apply a permalin
 const sass = require('metalsmith-sass')                      // convert Sass files to CSS using LibSass
 const hashAssets = require('metalsmith-fingerprint-ignore')  // add hash to specified files and ignores files that match a pattern
 
+const rollup = require('metalsmith-rollup')                  // used to build GOV.UK Frontend JavaScript
+const resolve = require('rollup-plugin-node-resolve')
+const commonjs = require('rollup-plugin-commonjs')
+
 // const debug = require('./debug')                          // debug plugin
 const navigation = require('./navigation.js')                // navigation plugin
 const modernizrBuild = require('./modernizr-build.js')       // modernizr build plugin