Improve footnote marks

[rich-iannone]

This improves the appearance of footnote marks for HTML output.

Fixes: #511

[jcheng5]

Instead of not having gt_footnote_mark on *, let's instead add an additional class gt_asterisk if it's an asterisk. This lets people continue to use gt_footnote_mark to select/style footnote marks.

[rich-iannone]

Instead of not having gt_footnote_mark on *, let's instead add an additional class gt_asterisk if it's an asterisk. This lets people continue to use gt_footnote_mark to select/style footnote marks.

Now done! Thanks for the input on this.