feat: override state(former "enviroment") values via command-line args (#644)

The addition of `--set k1=v1,k2=v2` and `--values file1 --values file2` was originally planned in #361.

But it turned out we already had `--values` for existing helmfile commands like `sync`. Duplicated flags doesn't work, obviously.

So this actually add `--state-values-set k1=v1,k2=v2` and `--set-values-file file1 --set-values-file file2`.

They are called "state" values according to the discussion we had at #640

Resolves #361

pkg/maputil/maputil.go

@@ -48,3 +48,28 @@ func CastKeysToStrings(s interface{}) (map[string]interface{}, error) {
 	}
 	return new, nil
 }
+
+func Set(m map[string]interface{}, key []string, value string) map[string]interface{} {
+	if len(key) == 0 {
+		panic(fmt.Errorf("bug: unexpected length of key: %d", len(key)))
+	}
+
+	k := key[0]
+
+	if len(key) == 1 {
+		m[k] = value
+		return m
+	}
+
+	remain := key[1:]
+
+	nested, ok := m[k]
+	if !ok {
+		new_m := map[string]interface{}{}
+		nested = Set(new_m, remain, value)
+	}
+
+	m[k] = nested
+
+	return m
+}