Introduce four-valued logic

[SimonBuxx]

In four-valued logic, in addition to the values high and low, there are values for things like unknown or high-resistance. We could go into that and define a useful set of logic values to implement into LogiJS.

Technically, this would call for changes in the combination logic at multiple points (what happens, when wires of different states connect, etc.) and the introduction of new colors or wire markers. Could be relatively easy to implement, but could also be way more difficult than I imagine at the moment.