add way to specify scrollerOffset from shortcode

[composerjk]

It would be nice to have a way to allow an implementor to change the scrollerOffset for a transcript in case they want the highlighted section to not be at the top of the window.

As I noted in #128:

I was just reminded that I had made a change (not included in the branches) to the default scrollerOffset from 0 to -160 so that the highlighted portions of the transcripts are more centered. Will open an issue to mention adding support of specifying that value.

The image in that issue also shows the placement of the highlight.

[maboa]

Yes. That sounds like a good idea 👍🏼

[maboa]

I think this might have to be first exposed in the Hyperaudio Lite Library.