Fix ElseIf for HTML mode

[MarSoft]

Previous elsif keyword is neither valid for Jade syntax (AFAIK) nor works with nodes.py Conditional class: elsif is not recognised as "next block" for if statement.

[kakulukia]

Since there are no updates to pyjade and its successor recently was deleted from pypi, I took the liberty to revive it: https://github.com/kakulukia/pypugjs

Version 5.0.1 has fixed the incompatibility with recent Django versions.

Please create another PR at my repo and will include that into the next verion.