Add potion effects to bosses when they spawn, implements #252.

With this feature you can add potions to the bosses when they spawn,
adding the ability to add strength to the boss for the first 30 seconds
making it harder for the first few seconds and once the effect wears
goes away the boss becomes easier. The syntax is in the same style as
potion items, except it uses either the potion effect enum name or the
potion effect id.

POTION_EFFECT:duration in seconds:amplifer

SLOW:360:4;INVISIBILITY:36000:0

If no duration is given, the plugin will default to sixty seconds. If
the amplifer was not provided, the plugin will default to zero. The
potion effect amplifcation starts at 0 being level 1.