Add option to interpret text as neither md nor html

[ddsjoberg]

There are a number of functions that default to treating the text input with gt::md(), and optionally can request md::html().

There could be cases where no interpreation is wanted, and we could add a third option text_interpret = c("md", "html", "none")

[ddsjoberg]

Once this is added, update the add significance stars source note to be interpreted with none