posterdown_memoir phased to posterdown_pdf

Fully-ish customised sizing of the poster and colours :partly_sunny:
diff --git a/R/posterdown_pdf.R b/R/posterdown_pdf.R
new file mode 100644
index 0000000..06f5bd7
--- /dev/null
+++ b/R/posterdown_pdf.R
@@ -0,0 +1,22 @@
+#' Posterdown PDF format (using 'memoir' document class)
+#'
+#' @inheritParams rmarkdown::pdf_document
+#' @param ... Additional arguments to \code{rmarkdown::pdf_document}
+#'
+#' @return R Markdown output format to pass to
+#'   \code{\link[rmarkdown:render]{render}}
+#'
+#' @examples
+#'
+#' \dontrun{
+#' library(rmarkdown)
+#' draft("MyArticle.Rmd", template = "posterdown_generic", package = "posterdown")
+#' }
+#'
+#' @export
+posterdown_pdf <- function(..., keep_tex = TRUE) {
+  inherit_pdf_document(...,
+                       keep_tex = keep_tex,
+                       template = find_resource("posterdown_pdf", "template.tex"),
+                       citation_package = "biblatex")
+}