atuin: add module

This commit adds a module for configuring atuin, a replacement shell
history program.

The module adds options for generating atuin's `config.toml` from Nix,
and options to enable atuin's integration for bash and zsh
(which will rebind history keys to open the atuin history).

modules/misc/news.nix

@@ -2214,6 +2214,13 @@ in
           'services.screen-locker'.
         '';
       }
+
+      {
+        time = "2021-10-05T20:55:07+00:00";
+        message = ''
+          A new module is available: 'programs.atuin'.
+        '';
+      }
     ];
   };
 }