Omitting REVEAL_MARGIN, REVEAL_MIN_SCALE, or REVEAL_MAX_SCALE causes a failure to export

(string-to-number (plist-get info :reveal-margin)) fails if the reveal_margin value is the default of -1. It might be better to use a smarter string-to-number implementation which only does the conversion if the input is not already a number. That said, the attached patch fixes the immediate problem.