Add support for transpiling ES6 Unicode regular expressions

https://github.com/mathiasbynens/regexpu
https://mathiasbynens.be/notes/es6-unicode-regex
Use HTTPS for the demo REPL

GitHub Pages offers HTTPS out-of-the box — there’s no reason not to use it.
Use the `files` directive in `package.json` instead of `.npmignore`

`.npmignore` is a blacklist, while `files` is a whitelist-based approach. The latter is much easier to maintain. See https://www.npmjs.org/doc/files/package.json.html#files for more info.

This patch does not affect the files that are included in the package.