Add ... arg to downloadButton

[bborgesr]

Added ability to pass arguments to the a tag function called inside downloadButton() and downloadLink(). Closes #986.

After seeing this referenced in another issue, this seemed like a very reasonable request that is not hard or risky to implement and doesn't break backward compatibility. Do you agree @wch, @jcheng5 ?

Demo app:

ui <- fluidPage( downloadButton("download", style = "background-color:#C9DD03"))
server <- function(input, output, session) {}
shinyApp(ui, server)