make slideshow width and autorotation configurable

assets/scss/snap-gallery.scss

@@ -30,6 +30,9 @@ $gap: var(--gap);
 }
 
 .snap-slideshow {
+  width: var(--slideshow-width);
+  margin: auto;
+
   // Hide all contained images except the first
   .snap-image:not(:first-child) {
     display: none;