Allow generating Gemini

[ibotty]

I'd like to generate Gemini (markup) from markdown files, ideally with the same library software I use already uses to generate HTML. Would you potentially accept a patch adding support (without additional dependencies of course)?

Of course it's a complicated by the fact that there not being a spec on how to translate various markdown constructs to gemini.

https://gemini.circumlunar.space/docs/specification.html section 5: The text/gemini media type

[marcusklaas]

This is certainly something that pulldown can be used for, but I believe it's not something that needs to be in the pulldown crate itself. Something similar was done in the pulldown-cmark-to-cmark crate, which uses pulldown to re-emit markdown, possibly after doing some transformations.