Add FireworkExplodeEvent

The FireworkExplodeEvent would be fired when a fireworks explodes, so that plugins developers could use the location of the explosion, or the firework itself, to change its color for example, or to act on the entities nearby.
It would also enable them to create firework-disabled areas, in which they would disable fireworks' explosions.
(I hope the file I submitted is correct)
I hope this event will be implemented, it would help the spell-plugins developers a lot!