Suggestion: Block whitelist for revert_on_unclaim

[galacticwarrior9]

Please explain your feature request to the best of your abilities:

Currently, revert_on_unclaim allows you to blacklist blocks from regenerating via the block_ignore option.

On servers where building in Wilderness is permitted, a very large number of blocks may need to be blacklisted to prevent player-made structures from regenerating. For such servers, it is preferable to simply have 'natural' blocks such as leaves, logs, grass, dirt and stone regenerate. This would be much simpler to do with a whitelist option that only allows the specified blocks to regenerate.