Add support for React classes in js2-imenu-extras

The already-present `js2-imenu-record-backbone-extend` works for [React] [] classes too, so this just renames it to `js2-imenu-record-backbone-or-react` and makes both `js2-imenu-record-backbone-extend` and `js2-imenu-record-react-class` aliases of it. And, of course, adds a specification for React to `js2-imenu-record-backbone-extend`.

[React]: http://facebook.github.io/react/index.html