Fix failure when item.msg is array

Author: Antiavanti

Diff for: lib/formatter.js

@@ -12,7 +12,7 @@ var cwd = process.cwd();
  */
 var formatRow = function(item) {
   // Drop newline symbol in error string
-  item.msg = item.msg.replace('\n', '');
+  item.msg = (Array.isArray(item.msg) ? item.msg.join(' ') : item.msg).replace('\n', '');
   // Using zero instead of undefined
   item.column = (item.column === undefined) ? 0 : item.column;
   // Drop `PUG:LINT_` prefix in error code