Add support for the `/y` flag (`RegExp.prototype.sticky`)

[mathiasbynens]

https://people.mozilla.org/~jorendorff/es6-draft.html#sec-get-regexp.prototype.sticky

This doesn’t affect parsing, so it seems fairly easy to do.

[adrianheine]

Since regjsparser doesn't actually handle the flags itself and y doesn't alter parsing, I don't think there's anything to do here.