Added 1.13- support, multiple item spawning and inv clearing
I had a friend with shitty hardware, wanted to use this on a 1.8.8 server so I added support for older versions that do not support PDC, so it should work back through 1.8 now.
I also noticed a limitation with items; I wanted to make a kit to give TNT and a flint & steel, but it only supported spawning one item before. So now if you create numeric subkeys, it will spawn each. Really, doesn't have to be numeric. Just so long as a 0 subkey exists, that is what it looks for.
I also added an inventory clear command - /kt clear