Make composite environment variable name optional

[ilyakaznacheev]

There are some cases when you can't change the environment variable name to the desired format.

For example, Heroku provides a PORT environment variable for you, you can't change the name.

But with the current library implementation, I can't structure my config files like I want to. Example:

type Config struct {
    Server ConfigServer `yaml:"server"`
}

type ConfigServer struct {
    Host string `yaml:"host"`
    Port string `yaml:"port" env:"PORT"`
}

The variable name library is looking for will be SERVER_PORT, not PORT, and I can't change that without restructuring my config structure (and config files!).

Please provide an option to turn off automatically provided env prefixes, to enable exact env names.

[cristaloleg]

(discussed on priv)
I will add param to env, so it will looks like: env:"NAME,exact" which means that exactly this name will be mapped on to this field.

and There will be an option to skip name (env:"-") in child fields.