Reimplemented materials.

* Materials are now read from materials.cfg, which can include
  additional metadata.
* Items reads blocks from materials.
* Removed all block types from items.txt.
* Many materials names have changed to better match the game/wiki.
* Materials now provides some centralized lists of blocks for
  heightmaps, vine attachments and more in the future.
* Wildgrowth now grows vines.

Issue #182
Issue #178