Support delegation

[SchulteMarkus]

Please support delegation.

package main

import (
	"fmt"
	"github.com/caarlos0/env"
)

type Foo struct {
	Food string        `env:"FOOD"`
}

type Bar struct {
	Baz string        `env:"BAZ"`
	Foo
}

func main() {
	bar := Bar{}
	err := env.Parse(&bar)
	if err != nil {
		fmt.Printf("%+v\n", err)
	}
	fmt.Printf("%+v\n", bar)
}

BAZ=1234 FOOD=4321 go run main.go

Is {Baz:1234 Foo:{Food:}}
Feature request {Baz:1234 Foo:{Food:4321}}

[caarlos0]

I like this idea.

Will think about it a little more and try to implement.

Thanks for the suggestion!

[caarlos0]

fixed on #36