Use werl instead of erl when executing iex.bat

This is more to start some discussion around this than necessarily the final solution for the problem. This commit sets an environment variable `useWerl`, which is then checked prior to executing `erl` in `elixir.bat`. If it is set to 1, `werl` is used instead of `erl`. This allows someone to override it and use `erl` for iex if they just want to use their native shell for `iex` (which still works, it's just missing some features like the CTRL+C and CTRL+G menus).

Thoughts?