exporting mongoose.Query.prototype.cache ;

Fantastic work thanks - A question and possibly due to my ignorance but to get this to work I needed to export the cache function 

```
module.exports.cache = mongoose.Query.prototype.cache ;
```

and then require it in the calling module

```
const  clearKey  = require("./cache").clearKey;
const cache = require('./cache').cache
```

Is this correct ?
