add new option for an custom path-like object#45

Sometimes we may work for a different fs, which requires different `path.sep` or `path.join()`. For instance, I am developing a nodejs program on windows, but the program is expected to be run on an linux. Then I need to inject [`path.posix`](https://nodejs.org/api/path.html#path_path_posix), instead of using the vanilla `path`, so that the results are returned as '/xxx/yyy` instead of `C:\\xxx\yyy`. It's far more flexible if klaw allowing setting a "mocked path object". :-)