Config support get environment variable

get environment variable if config item  has prefix "$ENV_" .
e.g.
```ini
[demo]
password = $ENV_MyPWD
```

config/config.go

@@ -44,6 +44,8 @@ package config
 
 import (
 	"fmt"
+	"os"
+	"strings"
 )
 
 // Configer defines how to get and set value from configuration raw data.
@@ -107,6 +109,24 @@ func NewConfigData(adapterName string, data []byte) (Configer, error) {
 	return adapter.ParseData(data)
 }
 
+const envKeySign = "$ENV_"
+
+// Getenv return environment variable if env has prefix "$ENV_".
+func Getenv(env interface{}) (string, bool) {
+	if env == nil {
+		return "", false
+	}
+
+	// Onley support string key.
+	if key, ok := env.(string); ok {
+		if len(key) > len(envKeySign) && strings.HasPrefix(key, envKeySign) {
+			key = strings.TrimLeft(key, envKeySign)
+			return os.Getenv(key), true
+		}
+	}
+	return "", false
+}
+
 // ParseBool returns the boolean value represented by the string.
 //
 // It accepts 1, 1.0, t, T, TRUE, true, True, YES, yes, Yes,Y, y, ON, on, On,