Allow for active port to be configured through PORT env var

Heroku uses dynamic port assignment, so this allows Heroku to connect to
the application using the port it's assigned. The default port of 3000
is still available for local development.

index.js

@@ -36,7 +36,7 @@ const httpServer = createServer(async (request, response) => {
   return response.end(template);
 });
 
-const PORT = 3000;
+const PORT = process.env.PORT || 3000;
 httpServer.listen(PORT, () => {
   console.info(`Server is running on port ${PORT}`);
 });