add conditional middleware example. Closes #5

examples/conditional-middleware.js

@@ -0,0 +1,33 @@
+
+var http = require('http');
+var koa = require('..');
+var app = koa();
+
+// logger
+
+function logger(next){
+  return function *(){
+    var start = new Date;
+    yield next;
+    var ms = new Date - start;
+    console.log('%s %s - %s', this.method, this.url, ms);
+  }
+}
+
+// sometimes it's useful to apply some
+// ad-hoc logic to enable middleware, for
+// example ignoring a logger on asset requests:
+
+app.use(function(next){
+  return function *(){
+    if (/(\.js|\.css|\.ico)$/.test(this.path)) {
+      yield next;
+    } else {
+      this.body = 'Hello World';
+      yield logger(next);
+      yield next;
+    }
+  }
+});
+
+app.listen(3000);