feat: add external handler through module options

lib/plugin.js

@@ -4,12 +4,15 @@ import { parse as parseCookie, serialize as serializeCookie } from 'cookie'
 import experiments from '<%= options.experimentsDir %>'
 const MAX_AGE = <%= options.maxAge %>
 
+const optimizeHandler = <%= serialize(options.handler) %>
+
 export default function (ctx, inject) {
   // Assign experiment and variant to user
   assignExperiment(ctx)
 
   // Google optimize integration
-  googleOptimize(ctx)
+  let handler = optimizeHandler || googleOptimize
+  handler(ctx)
 
   // Inject $exp
   inject('exp', ctx.experiment)