Request for convenience function

[aklitzke]

Might be nice to have something like:

use slog_scope::*;
new_scope!("key" => "value", || {
  info!("hi");
});

that is shorthand for:

use slog_scope::*;
with_logger(|logger| {
  scope(logger.new(o!("key" => "value")), || {
    info!("hi")
  });
});

[dpc]

I'd call it with_keys, but :+1: from me. Submit a PR and it will be there. :)