Add getInitOptions() function

Some developers would like to get the initial options such as currentLocale at some other files. Here you are.

FileA.js

import intl from 'react-intl-universal';
intl.init({ locales, currentLocale: "en-US" });

FileB.js

import intl from 'react-intl-universal';
const { currentLocale } = intl.getInitOptions();
console.log(currentLocale); // "en-US"