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

@@ -286,7 +286,7 @@ function runDevServer(host, port, protocol) {
   addMiddleware(devServer);
 
   // Launch WebpackDevServer.
-  devServer.listen(port, err => {
+  devServer.listen(port, host, err => {
     if (err) {
       return console.log(err);
     }
@@ -303,7 +303,7 @@ function runDevServer(host, port, protocol) {
 
 function run(port) {
   var protocol = process.env.HTTPS === 'true' ? "https" : "http";
-  var host = process.env.HOST || 'localhost';
+  var host = process.env.HOST || '0.0.0.0';
   setupCompiler(host, port, protocol);
   runDevServer(host, port, protocol);
 }