Add support for Shiny JS events

Hi Dean, long time no see ;) I can't say that enough: your package is awesome! The only thing I'm currently missing is the support for Shiny events. Right now I'm implementing king of a busy indicator which can be easily set-up with a custom JS code as shown in the example. Yet, it would be really great if something along the following lines would be possible - what do you think?

Cheers, Andrzej

library(shinyjs)

shinyApp(
  ui = fluidPage(
    useShinyjs(),
    ...
    div(id = "loader")
    ),
  server = function(input, output) {
    ...
    onshiny("busy", show("loader"))
    onshiny("idle", hide("loader"))
  }
)