expand environment variables from given stdin

config.cfg

a = "$GOPATH"
b = "$LANG"
c = "$NO_SUCH_ENVAR"

example_run.sh

# cat config.cfg | go run main.go
a = "/Users/doody/Workspace/gopath"
b = "en_US"
c = ""

expand_stdin.go

package main

import (
	"fmt"
	"io/ioutil"
	"os"
)

func main() {
	b, _ := ioutil.ReadAll(os.Stdin)
	fmt.Print(os.ExpandEnv(string(b)))
}

@doodyparizada does this support all the go template language spec? i need conditional support, etc.