fzf-zsh: init at unstable-2019-09-09

Simple `fzf` wrapper which provides a way nicer interface for ZSH's
<C-r> search. When using oh-my-zsh it can be enabled like this:

``` nix
{ pkgs, ... }: {
  programs.zsh = {
    enable = true;
    ohMyZsh = {
      enable = true;
      customPkgs = [ pkgs.fzf-zsh ];
      plugins = [ "fzf-zsh" ];
    };
  };
}
```