Implement reactive_counts option to the filter_settings

[gogonzo]

Could be done after #187

In the filter_settings we want the option reactive_counts = character(1) to enable/disable recalculation of the count labels. In cases when datasets are big, we would like to avoid the situation where all active filters are recomputed based on the filtered-values. When reactive_counts = FALSE the count bars will show only raw-data-counts as we used to present before.

Make sure that when reactive_counts = "none" initializes FilterState with the x_reactive = reactive(NULL)

Note: reactive_counts should be a character describing type of counts. IN the near future we will think more about "hierarchical" reactive counts which are straightforward now to implement (on the server side).