Allow users to manipulate request/response data before dumping

[jdufresne]

Dumping the request/response is useful for debugging, but that means the
data usually will end up in logfiles or other places. Since many
requests or responses will contain sensitive data (authentication
tokens, username, cookies etc), it is sometimes necessary to redact some
of that information before dumping it. This implementation provides a
way to supply your own sanitizer, or use the provided sanitizer which
will redact information in a number of known sensitive headers.

Closes #226

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This is a rebased version of the work started by @mortenlj in #227