Example of using map of structs in config, example of using env-separator

[Larleyt]

The docs say:
env-separator="<value>" - custom list and map separator. If not set, the default separator , will be used;
implying maps with arbitrary keys are supported. Yet, there's no example for it.

It is unclear how to assign env variable to a attribute of a struct which is a value of a map:

Let's say we have:
config.go:

Animals map[string]Animal  `yaml:"animals" env:"ANIMALS"`

Animal struct {
   Name: string `yaml:"name" env:"NAME"`
}

config.yaml:

animals: 
    dog:
        name: "Rocky"
    cat: 
        name: "Jack"

1. How do I provide names through env vars?
   Is it ANIMALS_DOG_NAME/ANIMALS_CAT_NAME or what?

2. What's env-separator is about?

@ilyakaznacheev