Initial PowerShell script addressing mix commas

This script simply preserves the intended formatting (comma placement) and calls the mix batch file.  PowerShell will call this script first if `mix` is given without an extension.  So doing things like `mix do deps.get, compile` functions as expected.  The next thing to fix is quoting, since quotes still get stripped.

Addresses [#2347](https://github.com/elixir-lang/elixir/issues/2347).