This is a proposal to make the current Go Logging package more extensible in a fully backawards
compatible way by allowing logger.Output() call a custom logging implementation to write logs
referencing this golang-nuts discussion.
To do this, we define a function type called OutputFn with the same signature as the current
Logger.Ouptut() and move its current implementation into a function called Loggger.DefOutputFn.
We introduce a mutex protected variable Logger.outputfn which by default points to Logger.DefOutputFn
but can be swapped for any other implementation by calling Logger.SetOutputFn()
For the standard logger, we introduce log.SetOutputFn() and log.SetDefOutputFn() to set and
reset custom loggers
The example file customlogger_test.go shows how we can use this to swap a logger that outputs logs in tab separated variable file format.