Add support for --werl in Windows bash-like shells

As per discussion on [elixir-lang-talk](https://groups.google.com/forum/?fromgroups=#!topic/elixir-lang-talk/bBiMB3hNJ8w). Only tested on Windows, but *should* not interfere on other OS-es.

TODO:
- [x] Test on a *nix (will test on my Mac)
- [x] Add doc to `bin/iex`
- [x] Add doc to `bin/elixir`