fix(genpass): check for presence of shuf command.

"shuf" is not a standard command on MacOS and requires installation of the brew coreutils package

plugins/genpass/genpass.plugin.zsh

@@ -75,6 +75,12 @@ genpass-xkcd() {
   # Generates a 128-bit XKCD-style passphrase
   # EG, 9-mien-flood-Patti-buxom-dozes-ickier-pay-ailed-Foster
   # Can take a numerical argument for generating extra passwords
+
+  if (( ! $+commands[shuf] )); then
+    echo >&2 "$0: \`shuf\` command not found. Install coreutils (\`brew install coreutils\` on macOS)."
+    return 1
+  fi
+
   local -i i num
 
   [[ $1 =~ '^[0-9]+$' ]] && num=$1 || num=1