Port / update to Nunjucks' filter block functionality

[ElChupacabra26]

Have you considered consolidating with the Nunjucks repo? I opened an issue re: the Nunjucks filter block functionality and this repo was mentioned on that thread. Just thought I'd throw it out there since you've already put in a good amount of work on this already. Anyways, if you're at all interested, here's a link to that issue.