Provide a more general method for saving HTML

Users shouldn't have to remember when to do htmlwidgets::saveWidget() vs htmltools::save_html() and the subtle differences between them.

Moreover, I want a way to save HTML tags without all the fuss of htmltools::save_html(), and I'm tired of waiting for https://github.com/rstudio/htmltools/issues/73

save_html <- function(x, file, selfcontained = TRUE, libdir = NULL, background = "white", knitrOptions = list()) {

  save_fun <- if (inherits(x, "htmlwidget")) {
    htmlwidgets::saveWidget
  } else if (inherits(x, "shiny.tag")) {
     save_tags
  } else { 
    stop("x must be an htmlwidget or an HTML tag list", call. = FALSE)
  }
    
  save_fun(x, file, selfcontained, libdir, background, knitrOptions)
}

Start by comparing htmlwidgets::saveWidget() with htmltools::save_html() and review the proposed save_html() wrapper, including its htmlwidget and shiny.tag cases. Done means one general saving method handles both supported input types and the listed options without requiring users to choose between the existing functions.