Support for vertical scroll

[weiw11]

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The only options currently are Left-Right or Right-Left depending on the format, but I'm sure there are others like me who prefers vertical scroll instead.

[4abhinavjain]

I use the reader to read and hear books (using Edge's text-to-speech engine). And it is always very problematic to keep track of the tts runner because of the layout. The epub.js suggests changing of layout should be possible. It will be a great help if someone can point how that can be done. Thanks