Fill in gaps according to light level

WORLDEDIT-3124 - Reported by CommodoreAlpha

When selecting a region and using either "/set " or "/replace ", there should be an additional, optional argument allowing you to specify the light-level required for the block to be replaced.

Examples can include "0" to indicate that only blocks with a light-level of 0 should be replaced (so, all solid blocks and all transparent blocks including air that have no light), ">7" to indicate that only transparent blocks with a light level greater than 7 should be replaced, and etc.

Although seemingly impractical, it could be a useful feature for nitpicky users who don't like air-pockets within anything they build (one reason being that it makes annoying ambient sounds). For instance, I would sometimes find myself terraforming via the "smooth-brush" only to find that I would occasionally leave behind an air-pocket in the hillsides I create.