More supports on class, style, and other attributes for UI

[atusy]

This PR makes following changes as written in NEWS.md

• Added class and ... arguments to *Output functions, so that these functions
  can take more controlls on the outputs' containers (generally div or span).

• Added style arguments to plotOutput and imageOutput, so that addtional CSS
  styles can be applied to them.

• Resolved inconsitency in class argument among downloadButton(), downloadLink() and icon().
  The class argument can be a character vector for downloadLink(), but not for others.
  Now all of them accepts a vector.

Usage example

To apply display: inline-block to plots to arrange them,
currently they have to be wrapped by additional div (see ui).

This PR removes additional div (see ui_new).

library(shiny)
library(ggplot2)

p <- ggplot2::qplot(1, 1)

server <- function(input, output, session) {
  output$a <- output$b <- output$c <- renderPlot(p)
}

# ui's are wrapped by div
ui <- fluidPage(
  plotOutput("a", width = 200, height = 200) %>% tags$div(style = "display: inline-block"),
  plotOutput("b", width = 200, height = 200) %>% tags$div(style = "display: inline-block"),
  plotOutput("c", width = 200, height = 200) %>% tags$div(style = "display: inline-block")
)

# ui's does not have to be wrapped by div anymore
ui_new <- fluidPage(
  plotOutput("a", width = 200, height = 200, style = "display: inline-block"),
  plotOutput("b", width = 200, height = 200, style = "display: inline-block"),
  plotOutput("c", width = 200, height = 200, style = "display: inline-block")
)

# compare
runGadget(ui, server)
runGadget(ui_new, server)

[atusy]

Note: I have signed to contributor agreement when I contributed to bookdown

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