add host argument for packager.

Summary:
Now packager only listen to "::", which is IPv6 "Any address".
It failed to run in IPv4 Environment.

defaults to undefined or empty string will fix this.

And I think it's necessary to let user define host by cli argument.

It's also for security reason. When working on a public network, it's much safer to listen with localhost instead of ::, which may let everyone in same network be able to get your code from debugger-ui.

recommit for facebook#1918, fixes facebook#2441
Closes facebook#5377

Reviewed By: svcscm

Differential Revision: D2842594

Pulled By: martinbigio

fb-gh-sync-id: 575944c5469dac80e99136a7903ea99f5339dba1

local-cli/server/runServer.js

@@ -44,7 +44,7 @@ function runServer(args, config, readyCallback) {
 
   const serverInstance = http.createServer(app).listen(
     args.port,
-    '::',
+    args.host,
     function() {
       attachHMRServer({
         httpServer: serverInstance,

local-cli/server/server.js

@@ -30,6 +30,10 @@ function _server(argv, config, resolve, reject) {
     command: 'port',
     default: 8081,
     type: 'string',
+  }, {
+    command: 'host',
+    default: '',
+    type: 'string',
   }, {
     command: 'root',
     type: 'string',