Allow users to mark certain machines as "do not use"

[andrewminer]

Some players may decide that certain machines are not worth the bother of creating or running, but still get crafting plans which call for those machines. They would like to be able to get a crafting plan which doesn't use them.

[jimbuck]

Along the same lines, what if you can specify how many furnace-like machines you have available. It seems like basic operations that are just time consuming (smelting sand into glass, etc) should be specified as a "pre-heat" step, so you can get it going then come back to it later.