Add support for React classes in js2-imenu-extras

Reuse the recorder function already present for Backbone extends. Rename
that function to js2-imenu-record-backbone-or-react and define
js2-imenu-record-backbone-extend and js2-imenu-record-react-class as
aliases of it.
Merge pull request #133 from johnmastro/imenu-react

Add support for React classes in js2-imenu-extras