Added support for multiple model directories

As the title says, support for multiple model directories. Could be a solution for #163

The `set_model_directory` and `get_model_directory` functions still work as they are proxied to `set_model_directories` and `get_model_directories`.

Note, view the diff with [`?w=1`](246/files?w=1), because the `<CR>`'s (carriage returns) are removed.