Adds redacting mutator extension

I've been wanting this feature from Sensu 0.11 for a while, but am not going to run beta software in production, so I created a mutator extension based off of the redaction feature of the upcoming release.

You can configure the keys to redact either through a client attribute `redact` or a setting snipping `redact`. If neither are configured, a basic set of keys is used. If both are set, the client's attribute is preferred.

This is my first time dealing with mutators as well as extensions, so if I'm off the mark anywhere, please let me know!