minor #497 feat(terser): enable cache and parallelism (Kocal)

Author: Lyrkan

lib/plugins/terser.js

@@ -18,7 +18,9 @@ const applyOptionsCallback = require('../utils/apply-options-callback');
  */
 module.exports = function(webpackConfig) {
     const terserPluginOptions = {
-        sourceMap: webpackConfig.useSourceMaps
+        sourceMap: webpackConfig.useSourceMaps,
+        cache: true,
+        parallel: true,
     };
 
     return new TerserPlugin(