ifies the number formatting:
        // - "h.v":   Horizontal . vertical slide number (default)
        // - "h/v":   Horizontal / vertical slide number
        // - "c":   Flattened slide number
        // - "c/t":   Flattened slide number / total slides
        //
        // Alternatively, you can provide a function that returns the slide
        // number for the current slide. The function should take in a slide
        // object and return an array with one string [slideNumber] or
        // three stri