add support for gemini (gemtext) links

[Misterio77]

Hey, thanks a lot for this plugin!

I've added support for gemtext-formatted links. This is a format pretty similar to markdown, and mostly used with the small-web gemini protocol.

There's some people using gemtext for their websites (gemtext can easily be converted to and from markdown or html), jekyll also works great when you want to have both a website and gemini capsule on the same project (my usecase).

This just basically adds a link_type and accompanying regex pattern for gemini-style links. Gemtext looks something like this:

# Heading 1

This is a paragraph

## Heading 2

Yo

=> ./file.gmi Link to my cool file

=> gemini://misterio.me Check out my website!

=> github.com

So the syntax is basically =>[whitespace]<url>[whitespace <Link title>].

I've also refactored the link_parts section a little bit.

Hoping this can be useful for other people, but I do understand if it's out of scope for this project and you prefer not to merge it.