Adding a method to merge config in the override section

This allows users to set configuration that would have precedence on env
variable. This is very useful when some post-processing is done on the
configuration.

viper.go

@@ -1278,6 +1278,23 @@ func (v *Viper) MergeConfigMap(cfg map[string]interface{}) error {
 	return nil
 }
 
+// MergeConfigOverride merges a new configuration within the config at the
+// highest lever of priority (similar to the 'Set' method). Key set here will
+// always be retrieved before values from env, files...
+func MergeConfigOverride(in io.Reader) error { return v.MergeConfigOverride(in) }
+func (v *Viper) MergeConfigOverride(in io.Reader) error {
+	if v.override == nil {
+		v.override = make(map[string]interface{})
+	}
+	cfg := make(map[string]interface{})
+	if err := v.unmarshalReader(in, cfg); err != nil {
+		return err
+	}
+	insensitiviseMap(cfg)
+	mergeMaps(cfg, v.override, nil)
+	return nil
+}
+
 // WriteConfig writes the current configuration to a file.
 func WriteConfig() error { return v.WriteConfig() }
 func (v *Viper) WriteConfig() error {

viper_test.go

@@ -1190,6 +1190,75 @@ func TestMergeConfig(t *testing.T) {
 	}
 }
 
+func TestMergeConfigOverride(t *testing.T) {
+	v := New()
+	v.SetConfigType("yml")
+
+	v.SetEnvPrefix("Baz")
+	v.BindEnv("fu")
+	os.Setenv("BAZ_FU", "test")
+
+	if err := v.ReadConfig(bytes.NewBuffer(yamlMergeExampleTgt)); err != nil {
+		t.Fatal(err)
+	}
+
+	if pop := v.GetInt("hello.pop"); pop != 37890 {
+		t.Fatalf("pop != 37890, = %d", pop)
+	}
+
+	if pop := v.GetInt("hello.lagrenum"); pop != 765432101234567 {
+		t.Fatalf("lagrenum != 765432101234567, = %d", pop)
+	}
+
+	if pop := v.GetInt32("hello.pop"); pop != int32(37890) {
+		t.Fatalf("pop != 37890, = %d", pop)
+	}
+
+	if pop := v.GetInt64("hello.lagrenum"); pop != int64(765432101234567) {
+		t.Fatalf("int64 lagrenum != 765432101234567, = %d", pop)
+	}
+
+	if world := v.GetStringSlice("hello.world"); len(world) != 4 {
+		t.Fatalf("len(world) != 4, = %d", len(world))
+	}
+
+	if fu := v.GetString("fu"); fu != "test" {
+		t.Fatalf("fu != \"test\", = %s", fu)
+	}
+
+	if err := v.MergeConfigOverride(bytes.NewBuffer(yamlMergeExampleSrc)); err != nil {
+		t.Fatal(err)
+	}
+
+	if pop := v.GetInt("hello.pop"); pop != 45000 {
+		t.Fatalf("pop != 45000, = %d", pop)
+	}
+
+	if pop := v.GetInt("hello.lagrenum"); pop != 7654321001234567 {
+		t.Fatalf("lagrenum != 7654321001234567, = %d", pop)
+	}
+
+	if pop := v.GetInt32("hello.pop"); pop != int32(45000) {
+		t.Fatalf("pop != 45000, = %d", pop)
+	}
+
+	if pop := v.GetInt64("hello.lagrenum"); pop != int64(7654321001234567) {
+		t.Fatalf("int64 lagrenum != 7654321001234567, = %d", pop)
+	}
+
+	if world := v.GetStringSlice("hello.world"); len(world) != 4 {
+		t.Fatalf("len(world) != 4, = %d", len(world))
+	}
+
+	if universe := v.GetStringSlice("hello.universe"); len(universe) != 2 {
+		t.Fatalf("len(universe) != 2, = %d", len(universe))
+	}
+
+	if fu := v.GetString("fu"); fu != "bar" {
+		t.Fatalf("fu != \"bar\", = %s", fu)
+	}
+}
+
 func TestMergeConfigNoMerge(t *testing.T) {
 	v := New()
 	v.SetConfigType("yml")