Randomized set

Can /set node node2 node3 and it will randomly choose between those
three.
Improved stacking

This stack / copy uses a direction vector, so it's not limited to only
along the X/Y/Z axis, and can go diagonally. This enables things like
building staircases.
Merge branch 'master' of github.com:cyisfor/Minetest-WorldEdit

Conflicts:
	worldedit/manipulations.lua
	worldedit_commands/init.lua