Add a spawner white/blacklist

Originally requested from _NotSoGoodGaming_ via https://dev.bukkit.org/bukkit-plugins/silkspawners/?comment=1406

> Any chance a blacklist* of mobs can be added into the config?
I'm looking for a way to block certain mobs from being allowed in spawners using /ss change <mob>, spawn eggs changing should be kept the same.
[...]
*A whitelist could also work.