Start the dev server at the specified host

Pass the host from environment variable as argument of the devServer's
listen function instead of a field of options object.
Set the default host to 0.0.0.0 instead of localhost.

packages/react-scripts/scripts/start.js

@@ -83,7 +83,7 @@ function run(port) {
   addWebpackMiddleware(devServer);
 
   // Launch WebpackDevServer.
-  devServer.listen(port, err => {
+  devServer.listen(port, host, err => {
     if (err) {
       return console.log(err);
     }