Fix #128: add `simple' key to inner/sectionpage

Adds a new simple style which only sets the section title centered on a section
page.

doc/metropolistheme.dtx

@@ -255,8 +255,9 @@ option on every sub-package accordingly.
   light grey or be transparent.
 }
 
-\DescribeOption{sectionpage}{none, progressbar}{progressbar}{
+\DescribeOption{sectionpage}{none, simple, progressbar}{progressbar}{
-  Adds a thin progress bar similar to the section progress bar underneath each frame title.
+  Disable section pages at all, typeset centered section title or add a thin
+  progress bar below the centered section title.
 }
 
 \DescribeOption{titleformat}{regular, lowercase, uppercase}{lowercase}{

source/beamerinnerthememetropolis.dtx

@@ -105,6 +105,7 @@
   /metropolis/inner/sectionpage/.cd,
     .is choice,
     none/.code=\@metropolis@sectionpage@none,
+    simple/.code=\@metropolis@sectionpage@simple,
     progressbar/.code=\@metropolis@sectionpage@progressbar,
 }
 %    \end{macrocode}
@@ -310,6 +311,22 @@
     % intenionally empty
   }
 }
+\defbeamertemplate{section page}{simple}{
+  \centering
+  \usebeamercolor[fg]{section title}
+  \usebeamerfont{section title}
+  \insertsectionhead\\
+}
+\newcommand{\@metropolis@sectionpage@simple}{
+  \setbeamertemplate{section page}[simple]
+  \AtBeginSection{
+    \ifbeamer@inframe
+      \sectionpage
+    \else
+      \frame[plain,c]{\sectionpage}
+    \fi
+  }
+}
 \defbeamertemplate{section page}{progressbar}{
   \centering
   \begin{minipage}{22em}