feat: add npmrc to file mapping (#86)

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##### Checklist
<!-- Remove items that do not apply. For completed items, change [ ] to
[x]. -->

- [x] `npm test` passes
- [ ] tests and/or benchmarks are included
- [ ] documentation is changed or added
- [x] commit message follows commit guidelines

##### Affected core subsystem(s)
<!-- Provide affected core subsystem(s). -->

- tests

##### Description of change
<!-- Provide a description of the change below this comment. -->

add `.npmrc` to file mapping

lib/init_command.js

@@ -29,6 +29,7 @@ module.exports = class Command {
       '_package.json': 'package.json',
       '_.eslintrc': '.eslintrc',
       '_.eslintignore': '.eslintignore',
+      _npmrc: '.npmrc',
       '_.npmignore': '.npmignore',
     };
   }

test/fixtures/simple-test/boilerplate/_npmrc

@@ -0,0 +1,3 @@
+registry=https://registry.npmjs.org/
+always-auth=false
+strict-ssl=false

test/init.test.js

@@ -32,6 +32,7 @@ describe('test/init.test.js', () => {
 
     assert(fs.existsSync(path.join(command.targetDir, '.gitignore')));
     assert(fs.existsSync(path.join(command.targetDir, '.eslintrc')));
+    assert(fs.existsSync(path.join(command.targetDir, '.npmrc')));
     assert(fs.existsSync(path.join(command.targetDir, '.npmignore')));
     assert(fs.existsSync(path.join(command.targetDir, 'package.json')));
     assert(fs.existsSync(path.join(command.targetDir, 'simple-app')));