Add quiet mode via --quiet flag

The implementation exactly matches how ESLint handles this
behavior in their CLI.

README.md

@@ -14,6 +14,14 @@ npm install eslintify
 browserify your_file.js -t eslintify
 ```
 
+### "quiet" mode
+
+Functionally equivalent to the [ESLint CLI](http://eslint.org/docs/user-guide/command-line-interface) flag `--quiet`: it causes warnings to be silently ignored. The default is for warnings to be outputted along with errors.
+
+```bash
+browserify your_file.js -t [ eslintify --quiet ]
+```
+
 ### "continuous" mode
 
 If you wish to get linting reports in the console but not break the build, enable "continuous mode" like so:

index.js

@@ -1,12 +1,18 @@
 var eslint = require('eslint');
 var through = require('through');
 var error = console.error.bind(console);
+var default_formatter = require.resolve('eslint-friendly-formatter');
+
+function hasCount(result) {
+    return result.errorCount || result.warningCount;
+}
 
 function lint(file, options) {
     options = options || {};
+    options.formatter = options.formatter || default_formatter;
 
     var cli = new eslint.CLIEngine(options);
-    var formatter = cli.getFormatter(options.formatter || require.resolve('eslint-friendly-formatter'));
+    var formatter = cli.getFormatter(options.formatter);
     var data = '';
 
     function write(buf) {
@@ -16,13 +22,16 @@ function lint(file, options) {
     function end() {
         var results = cli.executeOnText(data, file).results;
 
-        if (results.length && results.some(function (r) { return r.errorCount || r.warningCount; })) {
+        if (options.quiet) {
+            results = eslint.CLIEngine.getErrorResults(results);
+        } // filter out warnings
+
+        if (results.length && results.some(hasCount)) {
             error(formatter(results));
-        }
 
-        var errorResults = eslint.CLIEngine.getErrorResults(results);
-        if (errorResults.length && !options.continuous) {
-            this.emit('error', 'eslintify: linting error(s) detected.');
+            if (!options.continuous) {
+                this.emit('error', 'eslintify: linting error(s) detected.');
+            }
         }
 
         this.queue(data);
@@ -36,8 +45,13 @@ function lint(file, options) {
     var givenExtension = file.slice(file.lastIndexOf('.') + 1, file.length);
     var supportedExtensions = ['js', 'jsx', 'es6'];
 
-    if (options.extension) { supportedExtensions = supportedExtensions.concat(options.extension); }
-    if (options.extensions) { supportedExtensions = supportedExtensions.concat(options.extensions); }
+    if (options.extension) {
+        supportedExtensions = supportedExtensions.concat(options.extension);
+    }
+
+    if (options.extensions) {
+        supportedExtensions = supportedExtensions.concat(options.extensions);
+    }
 
     if (supportedExtensions.indexOf(givenExtension) !== -1) {
         return through(write, end);