Supporting a babel preset stage-x

[arunoda]

Right now, we have cherry-picked some post ES2016 features. I think it's better to support stage-3 or stage-2 presets instead.

With that, we could easily refer which features we support and so on.

[mxstbr]

Ref #22 why we chose the ones we did

[gaearon]

Hi, I know it’s a bit confusing but our strategy here is to use the syntax transforms that is either part of the standard, or that we also actively use at Facebook. Only this gives us confidence in using (and suggesting to use) experimental syntax.

[arunoda]

Sure. Got it.
Thanks @mxstbr for the reference. I couldn't find it.