Turn classoption in style-config into list

Pandoc uses a new, stricter YAML library since version 2.8. For fields
defined multiple times in a block, the old version used the last value,
while the new version rejects the whole block as invalid. This caused
the pandoc-scholar to ignore the contents of `style-metadata.inc.yml`.

journal-config/style-metadata.inc.yml

 ---
 documentclass: scrartcl
-classoption: abstract=on
-classoption: headlines=5
-classoption: headheight=5cm
+classoption:
+  - abstract=on
+  - headlines=5
+  - headheight=5cm
 lang: de-de
 fontsize: 9pt
 geometry: