Merge rnpm config with the cli config, step 1

Summary:
This is step one on merging the `rnpm` config with the `cli config`. The plan is to remove two sources of truth (rnpm package.json config and rn-cli Javascript config)

Rationale:
As of now, we have `rnpm` config, that used to live in the `local-cli/core/config/index.js` and the `rn-cli` config, living in `default.config.js` and `utils/Config.js`.

This PR moves all the things into one file, called `local-cli/core', simplifies things, enhances types (Config now has better types, added missing properties and fixed descriptions).

One notable addition is that users can now opt-in to override the commands that are loaded at the package level. Previously it was only possible to add a command by writing a plugin. Now, you can just tweak the `rn-cli.config.js`.

This is one of the several improvements I have on my roadmap, with better documentation for the CLI as well.
Closes https://github.com/facebook/react-native/pull/11564

Differential Revision: D4360095

fbshipit-source-id: feaec7c88df63e51cef1f9620c7eedeb738d3d00

local-cli/core/android/findAndroidAppFolder.js

@@ -0,0 +1,31 @@
+/**
+ * Copyright (c) 2015-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ */
+'use strict';
+
+const fs = require('fs');
+const path = require('path');
+
+/**
+ * @param  {String} folder Folder to seek in
+ * @return {String}
+ */
+module.exports = function findAndroidAppFolder(folder) {
+  const flat = 'android';
+  const nested = path.join('android', 'app');
+
+  if (fs.existsSync(path.join(folder, nested))) {
+    return nested;
+  }
+
+  if (fs.existsSync(path.join(folder, flat))) {
+    return flat;
+  }
+
+  return null;
+};