Use url option instead if href attribute

[bigfoot90]

Update to my last commit #383

This PR allows using button_link like that (shorten and more intuitive)

{{ button_link({
            'label': 'Test',
            'url': 'example.com',
}) }}

instead of

{{ button_link({
            'label': 'Test',
            'attr': {
                'href': 'example.com',
            },
}) }}

@florianeckerstorfer Please merge this small update