Add SetLevel method to the logger interface

I use your package for logging in a command line program. At the beginning it was enough for me to set the log level via command line paramets and initialize the logger via `stdlog.GetFromFlags()` but now that my program has grown I want to use configuration files as well. Thats why I need to be able to set the level of a logger during program execution. I thought maybe you want to merge that change into your base repo as well, even though I am aware of that this might break some stuff since it adds a new method to the interface