Added stacker_delay

Usually, external prop protections should be in charge of limiting
tools. Unfortunately, as many prop protections greatly differ and may
not easily allow editing, I've decided to include a time delay setting
for stacker that will prevent players from spamming the stacker tool too
quickly.

The default delay is 0, which is normal behavior (you can use stacker as
fast as you want). If you want there to be a 2 second delay between
using stacker, you can use stacker_delay 3 from the server console, or
stacker_set_delay 3 if you are a server admin.