The lua-discount dependency became a problem

https://github.com/leafo/sitegen/blob/ac9b7c199ed7adb32064b056a438cbaed8c71d23/sitegen-dev-1.rockspec#L20

The upstream project for lua-discount [has thrown in the towel](https://github.com/craigbarnes/lua-discount#status) over the discount v3 API changes, and v3 just hit me downstream on Arch Linux. Now nothing depending on lua-discount works an the maintainer has no intention of fixing it.