config: compare os.UserHomeDir and github.com/mitchellh/go-homedir, make a decision about integration

[kamilsk]

Motivation: configuration is needed for tools. For example, $HOME/.maintainer.

Research spf13/cobra@8eaca5f and https://golang.org/pkg/os/#UserHomeDir and compare it with https://github.com/mitchellh/go-homedir.

PoC

config.Load("config.name") // open $HOME/.tool/config.name with fallback to config.name in working directory