prevent plain npm

#22

build/npm/preinstall.js

@@ -0,0 +1,13 @@
+/*---------------------------------------------------------------------------------------------
+ *  Copyright (c) Microsoft Corporation. All rights reserved.
+ *  Licensed under the MIT License. See License.txt in the project root for license information.
+ *--------------------------------------------------------------------------------------------*/
+
+var win = "Please run '.\\scripts\\npm.bat install' instead."
+var nix = "Please run './scripts/npm.sh install' instead."
+
+if (process.env['npm_config_disturl'] !== 'https://atom.io/download/atom-shell') {
+	console.error("You can't use plain npm to install Code's dependencies.");
+	console.error(/^win/.test(process.platform) ? win : nix);
+	process.exit(1);
+}

package.json

@@ -12,6 +12,7 @@
   },
   "scripts": {
     "test": "node node_modules/mocha/bin/_mocha",
+		"preinstall": "node build/npm/preinstall.js",
     "postinstall": "npm --prefix extensions/csharp-o/ install extensions/csharp-o/ && npm --prefix extensions/vscode-api-tests/ install extensions/vscode-api-tests/"
   },
   "dependencies": {