Should I be able to load messages for language and territory like 'en-US'

Shouldn't I be able to load `en-US`. If not, then what is the proper way to load this data.

```javascript
var Globalize = require("globalize");
var cldr = require("cldr-data");

Globalize.load(cldr.all());
Globalize.loadMessages({
"en-US": { "common.legal.esrb.blood": "Blood" }
});
Globalize("en-US").formatMessage("common.legal.esrb.blood");
```

This gives the error: ```Error: E_MISSING_MESSAGE_BUNDLE: Missing message bundle for locale `en-US`.```