configurable `rel` attribute for links

[dumbmoron]

Currently, the rel attribute is always set to nofollow for external links: https://github.com/pngwn/MDsveX/blob/7861028758ac449999975d7ded82c834a555753c/packages/mdsvex/src/index.ts#L78

It would be nice if there was a way to change this behavior, e.g. not setting nofollow by default, or setting different values such as noreferrer.