Allow custom strip values, rather than defaulting to 1.

Unable to extract tar with multiple folders as root when the strip value is set to 1, would like the ability to set strip value when calling "extract" to allow for more flexibility of file structure within tarballs.

Not sure my solution is the best approach but hopefully something can be done to change this.
This should go before the callback. Also, it should be optional so you would need to include something like:

```js
if (_.isFunction(strip) {
  cb = strip;
  strip = 1;
}
```