Feature request - button to change page direction of ebook

Would it be possible to add a button to change the page direction?
This is useful for Japanese novels and books, where the text is normally read vertically right to left, with pages being turned from right to left.

This requires adding a parameter to the spine tag in contents.opf (around line 163 of saveEbook.js)

The default spine is defined as:
`''`

A right-to-left spine is defined as:
`''`

I have attempted implementing a button for the functionality but ended up breaking the locally unpacked extension for Chrome.