add exit command to CLI.

DeathSocketCLI/Commands.fs

@@ -10,6 +10,8 @@
 
         let test () = "Death Socket is working."
 
+        let exit () = Environment.Exit (Environment.ExitCode)
+
         let ``add-grid`` imgPath numRows numColumns pWidth colour newPath =
             try
                 printfn "[INFO.] Adding grid to image..."