Add a function for logging raw responses.

[my-name-is-lad]

Basically, that. Add a callback for logging or doing something else on raw responses.

[stevladimir]

httpConfigLogResponse name looks too specific. AFAIG it can be used far more than for logging, so should probably be named like httpConfigOnResponse. Or even httpConfigOnResponsePreview to be in consistence with httpConfigBodyPreviewLength option, which btw affects the behavior of this one.