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 */
})
```
cant lazy load only. Has to be a property of the exported thing
1.0.2
Merge branch 'master' of git://github.com/substack/tape

* 'master' of git://github.com/substack/tape:
  1.0.2
  cant lazy load only. Has to be a property of the exported thing
  Add the only property back to the exported test function