support some extra context in RequestLogging

[olliebun]

I'd like to be able to ask of the request logger:

include this value from the rack env in log messages

Or:

include this header from requests or responses in log messages

Or:

call this proc/lambda/block every time you log to let me customize log messages

This would allow us to nicely implement something like greensync/greensync-static-site#7

The base logger API exposed by this gem supports that kind of thing easily - but I can't think of a nice way to do this with the RequestLogging class as it is today.

[mdub]

Well, I'd figured one could always subclass RequestLogging, and override/extend the #request_details and #response_details methods. Does that do the trick?