Lookup env and toml field tags when flag is not given (#6)

_example/main.go

@@ -14,7 +14,8 @@ import (
 type cfgType struct {
 	Key1   string `toml:"key1"`
 	Key2   string `toml:"key2"`
-	Port   int    `toml:"-" flag:"port" env:"port"`
+	Host   string `toml:"host" flag:"server-host"`
+	Port   int    `toml:"-" flag:"port" env:"PORT"`
 	Secret string `env:"secret"`
 }
 
@@ -30,6 +31,7 @@ func main() {
 
 	filename := flag.String("config", "test.toml", "Config filename")
 	flag.Int("port", 8080, "Port to listen on")
+	flag.String("server-host", "default", "Server host address")
 	flag.Parse()
 
 	var cfg cfgType

_example/test.toml

@@ -1,3 +1,4 @@
 key1 = "123"
 key2 = "454"
 key3 = "key3val"
+host = "localhost"

config.go

@@ -17,6 +17,7 @@ import (
 const (
 	envTag  string = "env"
 	flagTag string = "flag"
+	tomlTag string = "toml"
 )
 
 // Load loads filepath into dst. It also handles "flag" binding.
@@ -89,8 +90,8 @@ func bindFlags(dst interface{}, metadata toml.MetaData) error {
 
 		useFlagDefaultValue := false
 		if !isFlagSet(tag) {
-			_, envHasKey := os.LookupEnv(tag)
-			if envHasKey || tomlHasKey(metadata, tag) {
+			_, envHasKey := os.LookupEnv(field.Tag(envTag))
+			if envHasKey || tomlHasKey(metadata, field.Tag(tomlTag)) {
 				continue
 			} else {
 				useFlagDefaultValue = true