Add gnu custom output format

This commit adds a custom output format that defined in [GNU coding
standard][1] like this:

    test.html:1.1: error: Doctype must be declared first.
    test.html:5.35: error: Tag must be paired, missing: [ </p> ], start tag match failed [ <p> ] on line 5.

With this output format, text editors (such as Vim) can easily read
output from HTMLHint.

[1]: https://www.gnu.org/prep/standards/standards.html#Errors

bin/htmlhint

@@ -46,7 +46,7 @@ program
     .option('-c, --config <file>', 'custom configuration file')
     .option('-r, --rules <ruleid, ruleid=value ...>', 'set all of the rules available', map)
     .option('-p, --plugin <file|folder>', 'load custom rules from file or folder')
-    .option('-f, --format <json|junit|checkstyle>', 'output messages as custom format')
+    .option('-f, --format <json|junit|checkstyle|gnu>', 'output messages as custom format')
     .option('-i, --ignore <pattern, pattern ...>', 'add pattern to exclude matches')
     .parse(process.argv);
 
@@ -176,8 +176,11 @@ function formatResult(hintInfo, format){
         case 'checkstyle':
             formatCheckstyle(hintInfo);
             break;
+        case 'gnu':
+            formatGNU(hintInfo);
+            break;
         default:
-            console.log('No supported format, supported format:json, junit, checkstyle.'.red);
+            console.log('No supported format, supported format:json, junit, checkstyle, gnu.'.red);
             process.exit(1);
     }
 }
@@ -275,6 +278,22 @@ function formatCheckstyle(hintInfo){
     }));
 }
 
+// format as GNU error format
+function formatGNU(hintInfo){
+  hintInfo.arrAllMessages.forEach(function (fileInfo) {
+    var file = path.relative(process.cwd(), fileInfo.file);
+    fileInfo.messages.forEach(function (message) {
+      console.log([
+        file,
+        message.line,
+        message.col,
+        " " + message.type,
+        " " + message.message
+      ].join(":"));
+    });
+  });
+}
+
 // hint all files
 function hintAllFiles(target, options, onFinised){
     var globInfo = getGlobInfo(target);