Enhance its platform compatibility

As pr in here [https://github.com/schneems/explain_shell/pull/2](https://github.com/schneems/explain_shell/pull/2)

the detection of the platform is necessary, however, i find that sh shell may be not provide us `OSTYPE`, so i change it to bash.

And in my opinion, it will be better writing these codes with python or ruby, it will be elegant and flexible