External dictionary

[fullpipe]

Fix #13

type MessageProvider interface {
  Get(lang language.Tag, msgPath string) (string, error)
}

provider := ... // my custom sql message provider

bundle, err := mf.NewBundle(
    mf.WithProvider(provider),
)

// or simple
bundle, err := mf.NewBundle(
    mf.WithYamlProvider(messagesDir), // embed yaml dictionary
)