Support "with-addons" build

[zpao]

We're still finalizing how it'll work, but the plan right now is a separate build that would drop-in replace react.js. So presumably this should just be another variant.

Details in facebook/react#370

[jtmalinowski]

Shouldn't this first appear in react-source?

[zpao]

Yep, it will in 0.5, just wanted to make sure this issue was filed.