> Can we have is_file and is_dir at the same time?
I added this flag to avoid tests like `if isinstance(resource, DirResource)` and have some `if resource.is_file` instead. So that in the future one could use another resource class that implements is_file or is_dir attributes.

> Maybe we should use a ENUM for that. type = FILE | DIRECTORY
With type attribute, the test would become `if resource.type is diecutter.resources.FILE`... is it easier to remember?