Add support for `sup` and `sub` HTML tags in `font.multi`

[cbouy]

Hi,

Currently, node and edge labels can support a limited number of font markups by enabling the font.multi property. Unfortunately <sup> and <sub> are not in the list of supported markup elements so it's not possible to use subscript/superscript in labels. I know there is a workaround with SVG to directly use HTML but it's very impractical to use. Would it be possible to add those 2 markups ?

[dudarboh]

I am working on a tool that would also benefit from this feature. It is quite handy to have in all kinds of scientific notations.