Support for text/gemini

[MCMic]

Hello,

It would be quite useful for me if retext could support gemini text format.

Mimetype is text/gemini, extension is usually .gmi or .gemini, and the specification is really simple, you can find it in section 5 of https://gemini.circumlunar.space/docs/specification.html

Basically it looks like this:

# Header 1
## Header 2
### Header 3
This is a paragraph
* This is an ordered list
* With two items
> This is a quote
=> https://gemini.circumlunar.space/docs/specification.html This is a link to the specification

You can also have preformatted text by surrounding it with lines starting by ```.

[mitya57]

Hi!

Unfortunately, I don't have enough time or interest to implement this myself, but Python-Markups module provides an API to implement any custom markups, and any such markup will be available in ReText too:

https://pymarkups.readthedocs.io/en/latest/custom_markups.html