Support for configurable headers...

[ansel1]

This project is handler middleware which extracts a configurable number of attrs from the record, and prints them as header fields before the message: https://github.com/dpotapov/slogpfx

I thought that was a clever idea, but would be more efficient to implement natively. I have a branch of console-slog which supports this type of function natively. In the following example, I configured the "logger" attr to be removed from the attr list, and inserted before the message as a header. (the value of "logger" is "console-slog" in this example...)

What do you think of this? Would this be a PR you would consider?