Currently it's rather difficult to install tmux plugins. The process involves two steps:
1. Specify the correct `pkg.tmuxPlugins` package in `environment.systemPackages`
2. Adding to the configuration file to instantiate the plugin.
This commit allows the user to specify a list of plugins under `programs.tmux.plugins`.
Update nixos/modules/programs/tmux.nix
Co-authored-by: Sandro <sandro.jaeckel@gmail.com>