[ANN] Collection of Gemini related Nix packages

[Emery]

I've created a collection of Gemini software that can be installed with
the Nix package manager. Most of these packages are in the main Nix
repositiory but there are collected here for convience. Since the Gemini
protocol is still experimental, it may be better to first submit packages
to a staging area like this.

=> https://git.sr.ht/~ehmry/gemini-flake

I don't have time to package every client and server, but I'm happy to
accept patches, and I can also review packages in the main repository.

Also, for anyone using NixOS who hasn't noticed, there is a configuration
module for molly-brown.
=> gemini://gemini.spam.works/~emery/nixos-hosting.gmi

The current dump of the package list:
```
git+https://git.sr.ht/~ehmry/gemini-flake?ref=master&rev=d67a4e923b9bf9e37b9b302ed4face2c491a1755
├───nixosModules
│   └───molly-brown: NixOS module
└───packages
    ├───aarch64-linux
    │   ├───amfora: package 'amfora-1.5.0'
    │   ├───asuka: package 'asuka-0.8.1'
    │   ├───av-98: package 'av-98-1.0.2dev'
    │   ├───castor: package 'castor-0.8.16'
    │   ├───gacme: package 'gacme-unstable-2020-07-07'
    │   ├───kristall: package 'kristall-0.3'
    │   ├───molly-brown: package 'molly-brown-unstable-2020-07-06'
    │   └───ncgopher: package 'ncgopher-0.1.5'
    └───x86_64-linux
        ├───amfora: package 'amfora-1.5.0'
        ├───asuka: package 'asuka-0.8.1'
        ├───av-98: package 'av-98-1.0.2dev'
        ├───castor: package 'castor-0.8.16'
        ├───gacme: package 'gacme-unstable-2020-07-07'
        ├───kristall: package 'kristall-0.3'
        ├───molly-brown: package 'molly-brown-unstable-2020-07-06'
        └───ncgopher: package 'ncgopher-0.1.5'
```