function to load js for using shinyFeedback
useShinyFeedback(feedback = TRUE, toastr = TRUE)
boolean: source in JS/CSS to use shinyFeedback functions (Default: TRUE)
boolean: source in JS/CSS to use showToast functions (Default: TRUE)
ui <- shinyUI(fluidPage( useShinyFeedback( feedback = TRUE, toastr = TRUE ), pageWithSidebar( headerPanel("Header"), sidebarPanel( ... ), mainPanel( ... ) ) ))