Add the only property back to the exported test function

Your lazy loading of the module.exports broke back compat with
the only property on the test object.

This brings the only method back so people can do

```js
var test = require("tape")

test.only("run this test", function (assert) {
  /* my test */
})
```

index.js

@@ -16,10 +16,16 @@ var nextTick = typeof setImmediate !== 'undefined'
 
 exports = module.exports = (function () {
     var harness;
-    return function () {
-        if (!harness) harness = createExitHarness();
+    var lazyLoad = function () {
+        if (!harness) {
+            harness = createExitHarness();
+            lazyLoad.only = harness.only;
+        }
+
         return harness.apply(this, arguments);
     };
+
+    return lazyLoad
 })();
 
 function createExitHarness (conf) {