More robust way to get path to file

[cboettig]

SO question

page.url doesn't have this data accurately for posts, since the date gets turned into directory. Reconstructing the original paths with gsubs is not ideal: https://github.com/cboettig/labnotebook/blob/a6ba6ee04fb023447155e3ef45fdc84eb995d832/_plugins/jekyll-labnotebook-plugins/raw_content.rb

need to find what variable available to generators that has access to this. thought page.slug might exist but doesn't seem so.