Support for variable/command substitution in dotenv files

[HartS]

The ruby dotenv has support for command substitution and basic variable substitution (but not yet expansion)

At the very least, variable substitution would be a nice to have in godotenv

[alexsasharegan]

I implemented the variable substitution in my copy-cat, learn-by-hacking version here: https://github.com/alexsasharegan/dotenv. Maybe the method can be ported to this lib's internals? I also needed var substitution for my existing environment files.

[joho]

I shall put it on my todo list