Add custom request filter (#693)

[profbiss]

trafficstars

Pull request to close issue #693

[barryvdh]

Do we need both variants? Just the secret seems enough?

[benyanke]

Any updates on when this might get pulled in?

For the record, I see a use for hide/vs/secrets. One hides things other devs wouldn't care about, and the other would be for things that matter, but just shouldn't have their content displayed.

[barryvdh]

I think I'm gonna solve this differently but still like the idea.