Support backup slides

Fix #126

demo/demo.tex

@@ -1,6 +1,7 @@
 \documentclass[10pt]{beamer}
 
 \usetheme{metropolis}
+\usepackage{appendixnumberbeamer}
 
 \usepackage{booktabs}
 \usepackage[scale=2]{ccicons}
@@ -302,6 +303,19 @@ or show \textbf{bold} results.\end{verbatim}
 
 \plain{Questions?}
 
+\appendix
+
+\begin{frame}[fragile]{Backup slides}
+  Sometimes, it is useful to add slides at the end of your presentation to
+  refer to during audience questions.
+
+  The best way to do this is to include the \verb|appendixnumberbeamer|
+  package in your preamble and call \verb|\appendix| before your backup slides.
+
+  \themename will automatically turn off slide numbering and progress bars for
+  slides in the appendix.
+\end{frame}
+
 \begin{frame}[allowframebreaks]{References}
 
   \bibliography{demo}

source/beamerouterthememetropolis.dtx

@@ -186,6 +186,18 @@
 %
 %
 %
+% \begin{macro}{appendix}
+%    \begin{macrocode}
+\AtBeginDocument{%
+  \apptocmd{\appendix}{%
+    \pgfkeys{%
+      /metropolis/outer/.cd,
+      numbering=none,
+      progressbar=none}
+    }{}{}
+}
+%    \end{macrocode}
+% \end{macro}
 % \subsubsection{Process package options}
 %
 %    \begin{macrocode}

source/beamerthememetropolis.dtx

@@ -188,7 +188,7 @@
         bg=palette primary.bg
       }
     }
-    \begin{frame}[c]{#1}
+    \begin{frame}[c,noframenumbering]{#1}
       \begin{center}
         \usebeamercolor[fg]{palette primary}
         \usebeamerfont{plain title}