i like this more than binding a new factory method inside the `doctest` function.
the `options` object could be a constructor (optional) parameter.
the constructor could work with `new`, but also be a simple method:
```javascript
var runner = doctest({module: true}):
runner('file.js', 'fine.js');
```
and i prefer this way. don't ask me why, i don't love java classes. anyway also with classes it's fine, but i prefer this `Doctest` interface to the `TestRunner` above.