Allow DOM element replacement in htmlOutput

[wch]

This PR adds a replace option to htmlOutput(). When set to TRUE, it tells Shiny that the output from renderUI should replace the container DOM element, instead of being placed inside of the container. This is useful in cases where an intervening <div> or <span> element isn't allowed, or will interfere with CSS selectors.

When replacement happens, the new element gets the old element's ID, and also gets assigned the classes "shiny-html-output shiny-html-replace-output". If renderUI returns something other than a single tag (like a tagList or bare text), it's not possible to replace the container element with the new content, preserving ID and and keeping those classes; in this case, the content will be placed inside the container element, as though replace=FALSE.

This example doesn't show the proper styling for the li (red text), because of the extra div in the DOM structure:

shinyApp(
  ui = bootstrapPage(
    tags$head(tags$style(HTML("ul > li { color: red; }"))),
    tags$ul(
      tags$li("static item"),
      uiOutput("item")
    )
  ),
  server = function(input, output) {
    output$item <- renderUI(tags$li("dynamic item"))
  }
)

With replace=TRUE, the li comes up with the correct styling:

shinyApp(
  ui = bootstrapPage(
    tags$head(tags$style(HTML("ul > li { color: red; }"))),
    tags$ul(
      tags$li("static item"),
      uiOutput("item", replace = TRUE)
    )
  ),
  server = function(input, output) {
    output$item <- renderUI(tags$li("dynamic item"))
  }
)

This addresses rstudio/shinydashboard#1.

[wch]

Closing because:

• Items updated with this code can only be updated once.
• To allow further updates, it requires changes to the output binding behavior.
• Changing the binding behavior is too risky for the relatively small benefit.