Compiling issue - Add support for windows

[technicalcapt]

Currently when compiling the app on windows, you'll get a compilation error like this:

** (ErlangError) Erlang error: :eacces
    erlang.erl:2217: :erlang.open_port({:spawn_executable, 'c:/Users/user/AppData/Roaming/npm/mjml.cmd'}, [:use_stdio, :exit_status, :binary, :hide, {:args, ["template.mjml", "-s"]}])