Ask if server should start at next open port

[glennreyes]

In addition to #86, I think it would be great, if it would open the next open port as a default, like exactly the same what create-react-app does. So instead of prompting

Error: listen EADDRINUSE :::4000
    at Object._errnoException (util.js:1024:11)
    at _exceptionWithHostPort (util.js:1046:20)
    at Server.setupListenHandle [as _listen2] (net.js:1351:14)
    at listenInCluster (net.js:1392:12)
    at Server.listen (net.js:1476:7)
 ...

... we should be able to decide if we want to open at 4001 instead.