Better support an empty "prefix"

[teepark]

It could use a special case for "":

package main

import (
    "fmt"
    "os"
    "github.com/kelseyhightower/envconfig"
)

var conf = struct {
    Field string `required:"true"`
}{}

func main() {
    os.Setenv("FIELD", "foobar")
    envconfig.MustProcess("", &conf)
    fmt.Println(conf.Field)
}

panic: required key _FIELD missing value