Make scoped custom rule package identifiers work with the --rules parameter (fixes #449).

Author: DavidAnson

.gitignore

@@ -28,6 +28,7 @@ build/Release
 node_modules
 # Keep it for test
 !test/custom-rules/relative-to-cwd/node_modules
+!test/custom-rules/scoped-package/node_modules
 
 # Optional npm cache directory
 .npm

markdownlint.js

@@ -199,7 +199,7 @@ program.parse(process.argv);
 
 function tryResolvePath(filepath) {
   try {
-    if (path.basename(filepath) === filepath && path.extname(filepath) === '') {
+    if ((path.basename(filepath) === filepath || filepath.startsWith('@')) && path.extname(filepath) === '') {
       // Looks like a package name, resolve it relative to cwd
       // Get list of directories, where requested module can be.
       let paths = Module._nodeModulePaths(processCwd);

test/custom-rules/scoped-package/node_modules/@scoped/custom-rule/package.json

@@ -0,0 +1 @@
+# Scoped Test

test/custom-rules/scoped-package/node_modules/@scoped/custom-rule/scoped-rule.cjs

@@ -535,6 +535,21 @@ test('Custom rule from node_modules package loaded relative to cwd', async t =>
   }
 });
 
+test('Custom rule with scoped package name via --rules', async t => {
+  try {
+    await execa(path.resolve('..', 'markdownlint.js'), ['--rules', '@scoped/custom-rule', 'scoped-test.md'], {
+      cwd: path.join(__dirname, 'custom-rules', 'scoped-package'),
+      stripFinalNewline: false
+    });
+    t.fail();
+  } catch (error) {
+    const expected = ['scoped-test.md:1 scoped-rule Scoped rule', ''].join('\n');
+    t.is(error.stdout, '');
+    t.is(error.stderr, expected);
+    t.is(error.exitCode, 1);
+  }
+});
+
 test('Custom rule from package loaded', async t => {
   try {
     const input = '# Input\n';