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. After using this module, [this repo passes the test for `strip` option](https://travis-ci.org/shinnn/decompress-tar/builds/32924147).