Allow customizing the logger middleware

[bensmrs]

My application runs two Dream servers and I’m interested in having both use the logger middleware. However, the way it’s currently implemented, both show the dream.logger source, making debugging a bit tedious. Could we have logger take an optional sub_log? The lowest-effort patch could look like let logger ?(log=log) next_handler request = ..., thus allowing injecting one’s own sub-logger into the logger middleware, without breaking the signatures too much.
WDYT?