permit post-processing callbacks bound to the stylus.render() 'end' e…

…vent

to extend callback flow and modify the css result.

lib/renderer.js

@@ -76,8 +76,12 @@ Renderer.prototype.render = function(fn){
     var compiler = new Compiler(ast, this.options)
       , css = compiler.compile();
 
-    this.emit('end', css);
-    fn(null, css);
+    if (!this.listeners('end').length) {
+      return fn(null, css);
+    }
+    this.emit('end', css, function(err, css) {
+      fn(err, css);
+    });
   } catch (err) {
     var options = {};
     options.input = err.input || this.str;