activate vertical-slides in default.yaml but keep the option to turn them off

resource/template/default.yaml

@@ -15,6 +15,7 @@ menu: true
 overview: true
 progress: true
 touch: true
+vertical-slides: true
 transition: "none"
 title: "|title|"
 test123: "test123"

test/decks/vertical-deck.md

 ---
 menu: true
 title: Vertical Slides
-vertical-slides: true
 ---
 
 # First slide

users-guide/decker-users-guide.md

@@ -209,7 +209,7 @@ control various aspects of the generated slide sets.
 :   mark overrflowing slides with a red border (defaults to `false`)
 
 `vertical-slides`
-:   allow vertical slides (defaults to `false`)
+:   allow vertical slides (defaults to `true`)
 
 ### Dictionary