Change arguments dict to accept them from current_app.url_map.iter_rules() directly

The current method of using `func.func_code.co_varnames` to get the list of arguments bring back a list that has more names than the functions have arguments (listed in this issue: https://github.com/acoomans/flask-autodoc/issues/5).

This change will use the `arguments` property for a rule if one exists.