Make target browsers configurable

Created by: gaearon

Imagine package.json like this:

  "browsers": {
    "development": "last 1 version",
    "production": "last 2 versions, > 5%"
  }

The format is the same as browserslist.

Then, we would use this information both for Babel and Autoprefixer.

Autoprefixer already supports this format of browser configuration.

We could teach babel-preset-env to honor it too, or we could write a helper that translates browserslist to its target.

This would solve #435 (closed) with some optional configuration.

It would make rebuilds faster and also make output less mangled without introducing sourcemaps, helping work around #139 (closed).