add a scubber

[phemmer]

Add some way to scrub fields out of a log entry (perhaps a more generic modify entry before relay).
For example, we log a http.Request, which can contain a Authentication: Basic header with private credentials in it. These should be scrubbed out before sent on to the handler. The field should still be in the result, but all characters should be replaced with x.

We should have a canned scrubber which looks for common fields that might contain private data: authentication, password, pass. Field matching should be case insensitive.