Allow builders doing nothing

[mase76]

Hi! Sometimes it is necessary to have builders doing nothing, but I don't want to unmount them. Maybe you could allow empty builders, an extractionof 0, or an on / off toggle.

[FaceDeer]

I was going to suggest putting an extremely large period value in the builder, like 100000, but I just tested that out and discovered that it crashes Digtron. :) I've fixed that crash now so this should be a viable workaround with the latest code, but I'll think a bit more about how to add something a bit less hacky for this (without making the interface even more cluttered, builders already have a pretty busy formspec).