Motivation for this change

Adds command-line interface to RSS-Bridge, so that it can be used without running it as a server.
It can be very useful for RSS feed applications such as newsboat (see "exec:" option).

It is made as a separate package because its CLI requires a php interpreter.
You can find official RSS-Bridge CLI documentation at https://github.com/RSS-Bridge/rss-bridge/wiki/Command-Line-Interface-(CLI)

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.