In 3.1.5 boolean arguments were allowed an optional argument
(true|false). This created a regression for uses of fused
short arguments, e.g. `-somyfile.html`, which was equivalent
to `-s -omyfile.html`, but now raised an error because
pandoc attempted to parse `o` as a boolean `true` or `false`.
This change adds a preprocessing step on the raw arguments
before they are sent to the option parser. In this preprocessing
step, `-somyfile.html` would be split into two arguments,
`-s` and `-omyfile.html`. The splitting happens when a
short boolean option is followed by another short option.
Closes#8956.
John MacFarlane
bec5429e4f