mangohud: add extraConfig option

[sebaszv]

Description

Added extraConfig option to append raw lines to the end of the
primary config.

This is a nice, simple addition to be able to add mainly comments,
along with anything else you'd like.

Checklist

• Change is backwards compatible.

• Code formatted with ./format.

• Code tested through nix-shell --pure tests -A run.all or nix develop --ignore-environment .#all using Flakes.

• Test cases updated/added. See example.

• Commit messages are formatted like

  {component}: {description}
  
  {long description}
  

  See CONTRIBUTING for more information and recent commit messages for examples.

• If this PR adds a new module

  • Added myself as module maintainer. See example.

Maintainer CC

@zeratax