Fix and improve `strip` option

* I added a test for [`strip` option](https://github.com/kevva/decompress-tar#strip).
* Currently `strip` option doesn't work correctly. Actually, [this repo fails the test for `strip` option](https://travis-ci.org/shinnn/decompress-tar/builds/32924026).  
    So I introduced [strip-dirs](https://github.com/shinnn/node-strip-dirs) module to fix this problem. It removes the leading path components like tar's `--strip-components` flag, and [well tested](https://travis-ci.org/shinnn/node-strip-dirs/jobs/32891230). After using this module, [this repo passes the test for `strip` option](https://travis-ci.org/kevva/decompress-tar/builds/32924528).
assert(!err);