Add a warning when omz's fzf plugin is loaded

[unixorn]

Per unixorn/zsh-quickstart-kit#177, it looks like you can't enable both oh-my-zsh's fzf plugin and this one.

Figure out a clean way to determine that the omz plugin is loaded and either don't load this plugin, or print an error message explaining that the user needs to disable the one from omz.