Default format to file extension

[jmgilman]

The README provides this example:

let
  configData = {
    "field1" = "value1";
    "field2" = true;
  };
in
nixago.lib.make {
  inherit configData;
  output = "config.json";
  format = "json";
  engine = nixago.engines.nix { }; # Optional as this is the default value
}

It seems slightly repetitive to specify an output file of config.json and then specify the format as json. We could potentially optimize for this case by defaulting the format option to the extension of the output option. This may be tricky in certain cases (i.e., yml or yaml?), but it should satisfy a number of easy scenarios.