There are far too many logging packages out there, with varying degrees of licenses, far too many features (colorization, all sorts of log frameworks) or are just a pain to use (lack of Fatalln()?). capnslog provides a simple but consistent logging interface suitable for all kinds of projects.
package main is the place where logging gets turned on and routedA library should not touch log options, only generate log entries. Libraries are silent until main lets them speak.
Still the job of main to expose these configurations. main may delegate this to, say, a configuration webhook, but does so explicitly.
main activates logging for its repository and any dependency repositories it would also like to have output in its logstream. main also dictates at which level each subpackage logs.
io.Writer composed with a formatter.Splitting streams is probably not the job of your program, but rather, your log aggregation framework. If you must split output streams, again, main configures this and you can write a very simple two-output struct that satisfies io.Writer.
Fancy colorful formatting and JSON output are beyond the scope of a basic logging framework -- they're application/log-collector dependent. These are, at best, provided as options, but more likely, provided by your application.
An object knows best how to print itself. Log objects can collect more interesting metadata if they wish, however, because text isn't going away anytime soon, they must all be marshalable to text. The simplest log object is a string, which returns itself. If you wish to do more fancy tricks for printing your log objects, see also JSON output -- introspect and write a formatter which can handle your advanced log interface. Making strings is the only thing guaranteed.