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 pull request #47 from cyisfor/master

Stacking in any direction
Documentation for `//stack2`, code style fixes, add author section to README.
Update `stack2` API documentation.
Oops, fix `//set`.
Slight optimization to //set

Just noticed I box the one type version in a list, to avoid testing
whether it's the one type version, but have to test for that to decide
whether to box it or not. Should shave like a whole 3ms from each //set
command.
CPS-ifying stack

Continuation Passing Style lets me use minetest.after, so the server
gets a chance to not hang in between every stack iteration. Could even
set minetest.after(1000,nextone) if you want to see it extend once every
second.
Fix worldedit_gui not working in Minetest 0.4.10.
Fix typo in README
Merge pull request #51 from cyisfor/master

I think that's the optimization you mentioned?
Coding style updates.
Oops, fix //set.
Rename the randomized //set to //mix, style update, document changes.
Fix //mix and improve normalize_nodename.
Add `sort_pos` to make code module independent.
Fix runtime error checking with lua* commands