Add option for adding a prefix

[onyxcodes]

Initial checklist

• I read the support docs
• I read the contributing guide
• I agree to follow the code of conduct
• I searched issues and couldn’t find anything (or linked relevant results below)

Problem

I thought it would be useful to be able to add a prefix to the generated slugs.
Had the case where on the same page there were multiple instances of rehype and I needed to make sure each slug generated, was unique throughout the page and prefixing them with an id related to the instance solved the problem.

Solution

Since I needed this feature, I've made a fork: 82d837c

Alternatives

I'm not sure there any alternatives, maybe adding some hidden content to the headings? Correct me if I'm wrong..

[wooorm]

See #13.
Your case is interesting and different though.

GH does some things like this for footnotes in comments (which are numbered, so, #fn-1). Where they prefix with a hash of the comment.
They never render headings with IDs from different sources on the same page though.

Interested in PRing the feature to this project?

[onyxcodes]

I'm not sure about the GH case you are talking about (care to point it out)?

Anyway, I'll open a PR.