Replacing WGUtil world height constants with configurable variables

[steyn9118]

I'd suggest to replace the "WGUtil.MIN_BUILD_HEIGHT" and "WGUtil.MAX_BUILD_HEIGHT" constants in WGMerge.java with variables available from plugin's config, because these constants are in fact equal to -32767 and +32767 respectively (as stated here) and not the actual world height limits, thus making the merged regions very tall which makes them quite hard to view in BlueMap.