Add "center" config option

This option allows you to specify your own initial center for a
tileset, which is useful if your map is extremely asymmetric or
you don't really care about what's around the spawn.

Future work needs to be done on the JS side in order to fix the
fromWorldToLatLng and friends, as they're currently off by -24 in X
and +24 in Z direction.

Closes #1350.

overviewer_core/tileset.py

@@ -589,7 +589,9 @@ class TileSet(object):
             imgextension=self.imgextension,
             isOverlay=isOverlay,
             poititle=self.options.get("poititle"),
-            showlocationmarker=self.options.get("showlocationmarker")
+            showlocationmarker=self.options.get("showlocationmarker"),
+            center=(self.options.get("center") or self.options.get("spawn")
+                    or [0, 64, 0])
         )
         d['maxZoom'] = self.options.get('maxzoom', self.treedepth)
         if d['maxZoom'] < 0: