set webpack-uglify-plugin ecma support based on browserlist

[viankakrisna]

given that #3618 and #3743 and #3644 recently merged to the next branch, we need a way to parse browserlist for uglifyjs es5-only support. This is done to ensure that no es6 dependency leaks through an es5 only environment.

Note:
need to verify that #2650 still works

[Timer]

We have a note [not issue] for tracking this here: https://github.com/facebookincubator/create-react-app/projects/3.

[Timer]

This setting doesn't actually restrict anything (very few places), so we might as well let everything parse.
We're also compiling node_modules now, so there's that.

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