feat: state values (#647)

This adds `values` to state files as proposed in #640.

```yaml
values:
- key1: val1
- defaults.yaml

environments:
  default:
  - values:
    - environments/default.yaml
  production:
  - values:
    - environments/production.yaml

```

`{{ .Valuese.key1 }}` evaluates to `val1` if and only if it is not overrode via the production or the default env, or command-line args.

Resolves #640

pkg/maputil/maputil_test.go

@@ -0,0 +1,61 @@
+package maputil
+
+import "testing"
+
+func TestMapUtil_StrKeys(t *testing.T) {
+	m := map[string]interface{}{
+		"a": []interface{}{
+			map[string]interface{}{
+				"b": []interface{}{
+					map[string]interface{}{
+						"c": "C",
+					},
+				},
+			},
+		},
+	}
+
+	r, err := CastKeysToStrings(m)
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+
+	a := r["a"].([]interface{})
+	a0 := a[0].(map[string]interface{})
+	b := a0["b"].([]interface{})
+	b0 := b[0].(map[string]interface{})
+	c := b0["c"]
+
+	if c != "C" {
+		t.Errorf("unexpected c: expected=C, got=%s", c)
+	}
+}
+
+func TestMapUtil_IFKeys(t *testing.T) {
+	m := map[interface{}]interface{}{
+		"a": []interface{}{
+			map[interface{}]interface{}{
+				"b": []interface{}{
+					map[interface{}]interface{}{
+						"c": "C",
+					},
+				},
+			},
+		},
+	}
+
+	r, err := CastKeysToStrings(m)
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+
+	a := r["a"].([]interface{})
+	a0 := a[0].(map[string]interface{})
+	b := a0["b"].([]interface{})
+	b0 := b[0].(map[string]interface{})
+	c := b0["c"]
+
+	if c != "C" {
+		t.Errorf("unexpected c: expected=C, got=%s", c)
+	}
+}