added languageData() for getting and setting language props at runtime

In my use case, certain language properties like the currency symbol are not strongly tied to the rest of the formatting configuration and can be changed at runtime.

*Usage*
``` javascript
numeral.languageData().currency.symbol = 'symbol';
numeral(100).format('$0,0') === 'symbol100' // true

numeral.languageData('en').currency.symbol = 'en-$';
numeral(100).format('$0,0') === 'en-$100' // true
```