Support 'structured logging'

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Many logging frameworks support 'structured logging'.
Client code passes as first parameter an object to the log method, and the fields of this object are copied to the log record.

E.g.:

var log = anylogger('my:logger')
log.info({ some: 'field'}, 'Some message'}

In combination with a JSON output, this should give output something like this:

{ "level": "info", "name": "my:logger", "some": "field", "message": ["Some message"]}