Options for YouTube player

Add a config option to change player style for dark.

Example:

// create timeline
          timeline = createStoryJS({
            type: 'timeline',
            embed_id: 'timeline',
            debug: false,
            youtubeTheme: "dark",
            source: { ... }
          });
Merge pull request #722 from t1gor/patch-1

Options for YouTube player