add prevent-block-place option

Prevents players from placing blocks outside the world border. Default
off.

This is for the PrisonPearl plugin, where if a player is imprisoned in a
chest placed far enough outside the world border it becomes impossible
to break the chest and free the player.

The BlockPlaceEvent listener is only registered if the option is
enabled.
Merge pull request #39 from gipsy-king/prevent_block_place

add prevent-block-place option