The package currently has two log formatters, along with the mechanisms to support them. If someone wants to have another formatter they need to add their code into the package (either here or in a fork).

I think it would be great to have a way for people to use the package and supply their own formatters. I've added support for custom log formatter here:
https://github.com/mlsquires/handlers/blob/feature/customloggers/handlers.go

Would you like a pull request?