Add support for loading erlang configuration files

Today, if you want to load an erlang configuration file when running an application, you need to specify the erl option: e.g. `elixir --erl "-config config" -S mix run --no-halt`.

This Pull Request add the functions `File.load_config/1` and `File.load_config!/1` that perform the same behaviour.

If this is accepted, it would be possible to do: `mix run --no-halt -c config`.