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)
}

首先比较 htmlwidgets::saveWidget() 与 htmltools::save_html() 并审查提议的 save_html() wrapper，包括其 htmlwidget 和 shiny.tag 情况。完成的标准是：使用一个通用的保存方法处理两种受支持的输入类型和所列选项，而不要求用户在现有函数之间进行选择。