feat: recursive traversal for image loading

[zSuperx]

Adds the --recursive flag, which simply keeps traversing directories (excluding . and ..) and adding their images to the loader.

Additionally modified .gitignore to ignore the output hellpaper binary, as it is platform-specific.

[zSuperx]

I will also note that by default, raylib is compiled to not decode .jpg images when opening via LoadImage(). To remedy this, I have been using the following flake.nix (shown below). If you wish to include a flake.nix like this to expose the hellpaper package and a development shell for Nix/NixOS, let me know and I can make a folllow-up PR that would just add a file similar to:

{
  inputs.nixpkgs.url = "github:nixos/nixpkgs";

  outputs =
    { nixpkgs, ... }:
    let
      system = "x86_64-linux";
      pkgs = nixpkgs.legacyPackages.${system};
    in
    {
      devShells.${system}.default = pkgs.mkShell {
        packages = with pkgs; [
          pkg-config # For implicit linking with third-party libraries (i.e. raylib)
          (raylib.overrideAttrs (
            self: {
              # override package to enable JPG image decoding
              cmakeFlags = self.cmakeFlags ++ [ "-DSUPPORT_FILEFORMAT_JPG=ON" ]; 
            }
          ))
        ];
      };
    };
}

This allows me to compile a version of hellpaper that supports JPG images without any code changes to hellpaper.c.

However, if you wish to exclude distro-specific build files, I can host it on my own fork instead.