Add gemini URI scheme

[prodrigestivill]

In order to detect properly gemini:// links I propose to add it as an unofficial URI scheme.

More info regarding this scheme at wikipedia and their official website.

[prodrigestivill]

If somebody requires it, as a workarround this lua filter or similar can be used using --lua-filter=filter.lua:

-- Fix gemini:// links
function Link(el)
  protopos = string.find(el.target, "/gemini://")
  if protopos then
    el.target = string.sub(el.target, protopos + 1);
  end
  return el
end