The following flags have been added:

• persistent: mobs will no longer despawn.
• invulnerable: mobs will not take damage (except from players in creative).
• silent: makes mobs silent.
• no-gravity: mobs will not be affected by gravity.
• glowing: adds a glowing effect to mobs.

To add any of these flags, simply include them in the command.
Example: /mobstack no-ai invulnerable glowing cat creeper...