Add transform config item
Allow blocks to transform into other blocks (grass->dirt is added by
default) when they slide.
Lots of new features:
- Added Slide-O-Tron wand
- Added WorldGuard support
- Less likely to leave floating blocks now
- Added block transforms (change block type when block slides, e.g.
grass->dirt)
Proper multi-world support
- Lots of parameters can now be set on a per-world basis with
"worlds.<world>.<key>"
- "slidy_world" config setting is gone; use "worlds.<world>.enabled"
boolean setting instead
- Added "horizontal_slides" per-world setting to allow/prevent blocks
sliding horizontally