Add reactive debounce and throttle functions

[jcheng5]

These are higher-order reactive functions to help slow down the effects of reactivity.

r <- reactive(input$chattyInput)

# r_throttled is a reactive expression with the same values as r, except it
# will update/invalidate no more than once every 500 millis
r_throttled <- r %>% throttle(500)

# Same, but will wait for r to be idle for at least 500 millis before
# updating/invalidating.
r_debounced <- r %>% debounce(500)

I also added options(device.ask.default = FALSE) to all the examples that use renderPlot. This prevents the Hit <Return> to see next plot from occurring while you're trying to run those apps.

• Add unit tests

[KZARCA]

Hi!
It would be great if the r argument could be an observer as well.