#' Return the HTML or image (png) of the javascript-rendered page in a local file
#'
#' The suggested use-case for this is rendering a widget
#'
#' TODO Test if container is running
#' TODO Enable passing in of an htmlwidget and use saveWidget
#'
#' @md
#' @param splash_obj Object created by a call to [splash]()
#' @param file_path Absolute path to a filename on the local host. **This only works with a locally running Splash instance started with [start_splash]().**
#' @param output either `html` or `png` to get the page content or an image capture
#' @param viewport View width and height (in pixels) of the browser viewport to render the web page. Format is "`<width>x<height>`". e.g. 800x600. Default value is 1024x768.