Add data attribute (data-is-bullet-footer) to footer

This way we can select it via CSS and do stuff like hiding it.
Rename footer_div_style to footer_div_attributes

This method now also includes the `data-is-bullet-footer` attribute, so
it'll be selectable by CSS.
Merge pull request #188 from rmehner/add_data_attribute_to_footer

Add data attribute (data-is-bullet-footer) to footer