Usage
ggslackr(plot = last_plot(), channels = Sys.getenv("SLACK_CHANNEL"),
scale = 1, width = par("din")[1], height = par("din")[2],
units = c("in", "cm", "mm"), dpi = 300, limitsize = TRUE,
api_token = Sys.getenv("SLACK_API_TOKEN"), file = "ggplot", ...)