Add Sub method on Builder, which populates the given struct starting …

…at the provided prefix.

config.go

@@ -69,11 +69,20 @@ func newBuilder() *Builder {
 // It panics if:
 //     * target is not a struct pointer
 func (c *Builder) To(target interface{}) error {
+	return c.decode(target, "")
+}
+
+// Sub behaves the same as To, however it maps configuration to the struct starting at the given prefix.
+func (c *Builder) Sub(target interface{}, prefix string) error {
+	return c.decode(target, prefix+c.structDelim)
+}
+
+func (c *Builder) decode(target interface{}, prefix string) error {
 	structPtr := reflect.ValueOf(target)
 	if structPtr.Kind() != reflect.Ptr || structPtr.Elem().Kind() != reflect.Struct {
 		panic("config: To(target) must be a *struct")
 	}
-	c.populateStructRecursively(structPtr, "")
+	c.populateStructRecursively(structPtr, prefix)
 	if c.failedFields != nil {
 		return fmt.Errorf("config: the following fields had errors: %v", c.failedFields)
 	}

config_test.go

@@ -70,6 +70,7 @@ func Test_Integration(t *testing.T) {
 	got := testConfig{
 		K: "hardcoded",
 	}
+	var sub D
 	want := testConfig{
 		A: 1,
 		B: "overridden",
@@ -90,9 +91,13 @@ func Test_Integration(t *testing.T) {
 
 	builder := From(file.Name()).From("nonexistfile").FromEnv()
 	gotErr := builder.To(&got)
+	_ = builder.Sub(&sub, "dog")
 	if !reflect.DeepEqual(got, want) {
 		t.Errorf("Integration: got %+v, want %+v", got, want)
 	}
+	if !reflect.DeepEqual(sub, want.D) {
+		t.Errorf("Integration: got %+v, want %+v", sub, want.D)
+	}
 	if gotErr == nil {
 		t.Errorf("Integration: should have had an error")
 	}

example_test.go

@@ -44,6 +44,19 @@ func Example() {
 	// [0.0.0.0 1.1.1.1 2.2.2.2] 3
 }
 
+func Example_subconfig() {
+	os.Clearenv()
+	os.Setenv("SUBCONFIG__IPWHITELIST", "0.0.0.0 1.1.1.1 2.2.2.2")
+
+	var c MySubConfig
+	config.FromEnv().Sub(&c, "SUBCONFIG")
+
+	fmt.Println(c.IPWhitelist, len(c.IPWhitelist))
+
+	// Output:
+	// [0.0.0.0 1.1.1.1 2.2.2.2] 3
+}
+
 func Example_defaults() {
 	os.Clearenv()
 	os.Setenv("DATABASE_URL", "production://")