Update to Pug v3.0.0

[brettinternet]

Pug is long out of date. Should we also consider making Pug a peer dependency rather than a required dependency?

[mnpenner]

I rewrote this plugin just now for Pug 3: https://gist.github.com/mnpenner/d201118e498172cfa8be69a646d0ece4

It's probably missing a few features and maybe doesn't handle edge cases perfectly, but it seems to be working fine for now.