xlsx and pptx support

[aantich]

Hi all, first of all, thank you for your great work - we use pandoc extensively as part of our platform, however we have really missed xlsx and pptx support for reading, so we added it (core haskell, not lua). Question is - is it something the community would be interested in reusing and if so how should we proceed? Thanks a lot.

[jgm]

Certainly we'd be interested in taking a look, if you'd like to make some PRs!

[aantich]

thanks, let us clean things up a bit and I'll make a PR.

[aantich]

So I created a PR, some checks are failing mostly with unused imports warnings as far as I could see, but when I build pandoc on my system there are quite a bit of those in pre-existing code. We can clean up to the best of our ability or if you have any other suggestions / questions - please share! Thanks a lot.