Add block helper support to jade extractor

[monitz87]

I have noticed that block helpers are yet to be extracted from jade files. In fact, at the end of the jade handler in the code there is a comment that indicates exactly that. I am just posting this as a reminder because I didn't find the issue and comments serve as poor reminders

Cheers

[gadicc]

Thanks again, and you're right again :) I'll happily take a PR for this from one of our Jade users.