Catch-all formatter callback for arbitrary tags

[sgrimm-sg]

This allows the caller to apply custom formatting to HTML tags other than the ones in the predefined list. The default behavior remains the same.

Example use case: insert line breaks before <div> tags.

[gangstead]

@mlegenhausen this would replace #154 and #138

[KillyMXI]

Version 6 now provides a somewhat different approach that allows to solve the same issue. This is no longer needed.

Thanks for the effort though.