Add description for Unicode regular expression feature

* I added a comment about [`regexpu` option](https://github.com/esnext/esnext/blob/fbfba30fb31893bd03cedb73fa97b91fa0b6dd01/lib/index.js#L91-L93) to `index.js`.
* I added a description about [Unicode regular expressions](https://github.com/lukehoban/es6features#unicode) to the README.
* I replaced the link of [es6-computed-property-keys](https://github.com/vslinko/es6-computed-property-keys) with [es6-computed-properties](https://github.com/DmitrySoshnikov/es6-computed-properties)'s. Current version (v0.12.0) doen't depends on es6-computed-property-keys.