Special-case use of HTML tags for converting `<sub>` / `<sup>`

[jsm28]

Special-case use of HTML tags for sub_symbol and sup_symbol. In particular, this allows setting sub_symbol='<sub>', sup_symbol='<sup>' to use raw HTML in the output when converting subscripts and superscripts.

[chrispy-snps]

@jsm28 - would you consider removing the *_after configuration options and automatically using the closing tag variant if the opening option is an HTML tag?

[jsm28]

I've revised things as suggested to handle HTML tags automatically instead of having separate configuration options.

[chrispy-snps]

Thank you @AlexVonB and @jsm28!