Lazy fields

It would be nice to have an optional ``WithLazyFields(key string, func() interface{})`` method to calculate field value only when it's retrieved with ``Get(key string)`` method.

I.e. some entries may be discarded due to log level, but they are still evaluated.

A real life example is dumping http requests/responses on debug level. It requires much more memory for each request to process, and it's painful to have some debug switches to turn it on/off on local/production system.