configurable rel attribute for links

[dumbmoron]

Currently, the rel attribute is always set to nofollow for external links:

MDsveX/packages/mdsvex/src/index.ts

Line 78 in 7861028

.use(external, { target: false, rel: ['nofollow'] })

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.