Update lib for es6

[benkeen]

Boy, this has gotten out of date! Functionally it's just fine & works correctly, but there are tons of things that can now be improved. It's a bit painful editing the old es5 code.

• move to es6 + babel.
• import the data-country-region JSON via webpack import rather than the template thing we're using right now.
• UMD is still useful of course, but a plain es6 export would be nice. Especially with the custom generation option to create a smaller lib with only the countries you're interested in.

Not sure when I'll get to this, but it anyone else wants to take the reigns...