FR: Allow Configuration of Value Used for States

[rehashedsalt]

Specifically, I'd like to be able to define which of a module's format replacements is used to set the thresholds for states.

The use case is pretty simple: I have a memory module that uses states to go from hidden -> neutral -> yellow -> red as memory usage is driven higher, and I would like to do the same for swap usage. There is no inbuilt functionality to support this right now -- the memory module is hardcoded to use memory usage percentage for states.