Allow user to define what node_modules to transpile down to ES5

[seivan]

This is regarding the issue with packages that aren't in ES6

There seems to be a binary choice here were one is forced to eject or pick one of the many solutions (submodule in src/ or fork) to get the package in ES5.

It would be nice if create-react-app started allowing for small hooks without a full blown ejection.
In this case, it be nice to allow what packages would need to be transpiled down to ES5 since they are distributed in ES6.

The main argument I've been reading is that it might open up a whole avenue of bugs since you're deviating from the version the author offers, but that shouldn't matter if it's something one opts into.

Relevant discussion #1125
This comment could be fixed by using something like:

exclude: /node_modules\/(?!(USER_DEFINED_PACKAGE_1| USER_DEFINED_PACKAGE_2)\/).*/

From babel/babel-loader#171 (comment)

[gaearon]

I don't think we'll allow this in particular.

If we run Babel on node_modules, we should only run the subset of plugins related to JS language features (and not JSX/Flow, for example).

We have an existing issue to track that work: #1125.

[gaearon]

FYI, we're starting the work to compile deps with babel-preset-env in Create React App: #3776

Let us know if you have feedback about how this should work.