add feature: threshold_count (fixes #1015)

I've run this for our use case, and it works nicely.

Ah, I see I missed to add `thresholdVisualBorderWidth` here.. @torkelo: do you want all possible options to be populated here? (it works just well without.. so I see it more as documentation over which options there are, and in this case, to be consistent with all the other options being there..).