Merge branch 'develop' into css-variables

# Conflicts:
#	demo/dist/demo.css
#	demo/index.html
#	dist/plyr.css
#	gulpfile.js
#	package.json
#	yarn.lock

src/sass/plyr.scss

@@ -45,6 +45,7 @@ $css-vars-use-native: true;
 @import 'states/fullscreen';
 
 @import 'plugins/ads';
+@import 'plugins/previewThumbnails';
 
 @import 'utils/animation';
 @import 'utils/hidden';