gistr: Work with 'GitHub' 'Gists'

#' @param files Files to upload. this or \code{code} param must be passed
#' @param description (character) Brief description of gist (optional)
#' @param public (logical) Whether gist is public (default: TRUE)
#' @param browse (logical) To open newly create gist in default browser 
#' (default: TRUE)
#' @param code Pass in any set of code. This can be a single R object, or 
#' many lines of code wrapped in quotes, then curly brackets (see examples 
#' below). this or \code{files} param must be passed
#' @param filename Name of the file to create, only used if \code{code} 
#' parameter is used. Default to \code{code.R}
#' @param knit (logical) Knit code before posting as a gist? If the file 
#' has a \code{.Rmd}  or \code{.Rnw} extension, we run the file with 
#' \code{\link[knitr]{knit}}, and if it has a \code{.R} extension, then 
#' we use \code{\link[rmarkdown]{render}}
#' @param knitopts,renderopts (list) List of variables passed on to 
#' \code{\link[knitr]{knit}}, or \code{\link[rmarkdown]{render}}
#' @param include_source (logical) Only applies if \code{knit=TRUE}. Include 
#' source file in the gist in addition to the knitted output.
#' @param imgur_inject (logical) Inject \code{\link[knitr]{imgur_upload}} 
#' into your \code{.Rmd} file to upload files to \url{https://imgur.com/}. 
#' This will be ignored  if the file is a sweave/latex file because the 
#' rendered pdf can't be uploaded anyway. Default: FALSE
#' @param ... Further args passed on to \code{\link[crul]{verb-POST}}