Golang : How to deal with configuration data?

A friend ask what are the ways to deal with configuration data in Golang. Couple of ways that I can think up are :

1. The simplest. Read configuration data from key-value pair file. See example on Golang Key-Pair value file.

2. Read data from YAML file with goyaml.

3. TOML - see https://github.com/BurntSushi/toml

4. Read from environment variables.

5. Hard code into the program source code. Which is not advisable..but if you're writing a small program or microservice..then it is ok. It will make life easier for future maintenance.

UPDATE :

1. Thanks to Mark for pointing out the Viper package. https://github.com/spf13/viper. From the Viper's documentation - "When building a modern application, you don’t want to worry about configuration file formats; you want to focus on building awesome software. Viper is here to help with that."

Reference :

https://www.socketloop.com/tutorials/golang-read-data-from-config-file-and-assign-to-variables