Add logging options to config

The library generates logging output when files are closed or recreated. I didn't want to display this to the user of my program so I added extra options to modify the way `tail` logs.

There's `Config.Logger`, allowing you to set your own Logger (using logfile, or other prefix or flags).
And there's `Config.DisableLogging`, which disables logging all together..
This doesn't change anything for existing users, as Config.Logger is set to the `log` package's default logger (`Log.std`) when it's nil.

I hope this code will be useful for anyone else out there :smile: