Delete slide options that are not defined

Reveal.js doesn't set the default value of an option in the provided config object
if the key is set with "undefined" as value. This leads to a broken slide mode,
because some critical settings are missing.

Signed-off-by: Tilman Vatteroth <tilman.vatteroth@tu-dortmund.de>

public/js/slide.js

@@ -124,6 +124,12 @@ var options = {
   width: meta.slideOptions.width
 } || {}
 
+for (const key in options) {
+  if (options.hasOwnProperty(key) && options[key] === undefined) {
+    delete options[key]
+  }
+}
+
 const view = $('.reveal')
 
 // text language