Add support for shiny::bindCache

[wch]

To be merged after ramnathv/htmlwidgets#391.

This makes renderPlotly work with shiny::bindCache in the dev version of shiny. For example:

library(shiny)
library(plotly)

ui <- fluidPage(
  selectizeInput(
    inputId = "cities", 
    label = "Select a city", 
    choices = unique(txhousing$city), 
    selected = "Abilene",
    multiple = TRUE
  ),
  plotlyOutput(outputId = "p")
)

server <- function(input, output, ...) {
  output$p <- renderPlotly({
    message("Computing p: ", paste(input$cities, collapse = ", "))
    Sys.sleep(2)
    plot_ly(txhousing, x = ~date, y = ~median) %>%
      filter(city %in% input$cities) %>%
      group_by(city) %>%
      add_lines()
  }) %>% 
    bindCache(input$cities)

}

shinyApp(ui, server)