feat(config): Add custom config for init private flag (yarnpkg#4377)

**Summary**

Here is a small custom; I add this because it relates to https://yarnpkg.com/en/docs/cli/init#toc-setting-defaults-for-yarn-init. I discovered this was necessary while writing introduces for this flag.

**Test plan**

New init and config tests.

__tests__/commands/__snapshots__/init.js.snap

@@ -1,5 +1,16 @@
 // Jest Snapshot v1, https://goo.gl/fbAQLP
 
+exports[`init-config 1`] = `
+Object {
+  "author": "Kittens McKitty <[email protected]> (https://yarnpkg.com)",
+  "license": "BSD",
+  "main": "index.js",
+  "name": "init-config",
+  "private": true,
+  "version": "0.0.0-alpha",
+}
+`;
+
 exports[`init-github 1`] = `
 Object {
   "license": "MIT",

__tests__/commands/init.js

@@ -84,6 +84,29 @@ test.concurrent('init --yes --private should create package.json with defaults a
   );
 });
 
+test.concurrent('init should use init-* configs when defined', (): Promise<void> => {
+  return buildRun(
+    ConsoleReporter,
+    fixturesLoc,
+    (args, flags, config, reporter, lockfile): Promise<void> => {
+      return runInit(config, reporter, flags, args);
+    },
+    [],
+    {yes: true},
+    'init-config',
+    async (config): Promise<void> => {
+      const {cwd} = config;
+      const manifestFile = await fs.readFile(path.join(cwd, 'package.json'));
+      const manifest = JSON.parse(manifestFile);
+
+      // Name is derived from directory name which is dynamic so check
+      // that separately and then remove from snapshot
+      expect(manifest.name).toEqual(path.basename(cwd));
+      expect({...manifest, name: 'init-config'}).toMatchSnapshot('init-config');
+    },
+  );
+});
+
 test.concurrent('init using Github shorthand should resolve to full repository URL', (): Promise<void> => {
   const questionMap = Object.freeze({
     name: 'hi-github',

__tests__/fixtures/init/init-config/.yarnrc

@@ -0,0 +1,6 @@
+init-private true
+init-license BSD
+init-version "0.0.0-alpha"
+init-author-name "Kittens McKitty"
+init-author-email "[email protected]"
+init-author-url "https://yarnpkg.com"

src/cli/commands/init.js

@@ -96,7 +96,7 @@ export async function run(config: Config, reporter: Reporter, flags: Object, arg
     {
       key: 'private',
       question: 'private',
-      default: '',
+      default: config.getOption('init-private') || '',
       inputFormatter: yn,
     },
   ];
@@ -129,7 +129,7 @@ export async function run(config: Config, reporter: Reporter, flags: Object, arg
     }
 
     if (def) {
-      question += ` (${def.toString()})`;
+      question += ` (${String(def)})`;
     }
 
     let answer;