Add ECMAScript module support

[nstringham]

This PR solves the same problem as #37 but in a different way

this PR also fixes #89

this PR allows users to do

import emojiRegex from 'emoji-regex';

or

const { default: emojiRegex } = await import('emoji-regex');

or

const emojiRegex = require('emoji-regex');

it does this by adding index.mjs which is like index.js except that is an es module instead of a commonjs module

[mathiasbynens]

Thanks!