Expanding variable value

[kameshsampath]

Say I have .env like

MY_USERNAME=${LOCAL_DEVELOPER_USERNAME}

The LOCAL_DEVELOPER_USERNAME is host environment variable, I wish the value of that to be expanded using the host environment variable value with possible overrides if defined locally in the environment file.

Let me know if you are interested in accepting a pull for the same as I have this logic working https://github.com/kameshsampath/godotenv/blob/issue-189/godotenv.go#L339-L347