feat: add update command

crates/gitnow/include/shell/zsh.sh

@@ -1,9 +1,16 @@
 function git-now {
+  # Run an update in the background
+  (
+    nohup gitnow update > /dev/null 2>&1 &
+  )
+
+  # Find the repository of choice
   choice=$(gitnow  "$@" --no-shell)
     if [[ $? -ne 0 ]]; then
     return $?
   fi
-  
+
+  # Enter local repository path
   cd "$(echo "$choice" | tail --lines 1)"
 }