Added an option to enter your own zoom level. Use -z to set the map at a
particular zoom level. Zoom levels define the width and height in tiles
of the highest zoom level, each new zoom level is twice as wide and
tall. (z=6 -> 2^6 tiles wide and tall)
Implemented tile re-arrangement on map expansion. Now most tiles will
get re-used if your map needs another zoom level! No longer does it need
to re-generate everything.
No longer creates empty directories for tiles, only creates directories
if needed.
Fixed some minor off-by-one logic (and the code that canceled it out to
make it work)
When the world expands and the Overviewer is forced to create a new zoom
level, use to be all tiles needed to be regenerated. Now that the world
center is always anchored, I can do some simple renaming of folders so
now when a world expands, tiles are re-used and it's very quick!
