Named exports

[Rich-Harris]

Re @Victorystick's suggestion on rollup/rollup#269 (comment) – it'd be nice to import a single field from e.g. package.json. Right now, this plugin only generates a default export:

export default {
  "name": "my-neato-package",
  "version": "1.33.7",
  "main": "dist/my-neato-package.umd.js",
  ...
};

We could easily generate named exports too:

export var name = "my-neato-package";
export var version = "1.33.7";
export var main = "dist/my-neato-package.umd.js";

export default {
  name: name,
  version: version,
  main: main,
  ...
};

That way, import { version } from './package.json' wouldn't include unnecessary gubbins.

It gets slightly tricky with key names that aren't valid identifiers, like jsnext:main. Do we ignore them, or turn them into valid identifiers?

export var jsnext_main = 'dist/my-neato-package.es6.js';

export default {
  'jsnext:main': jsnext_main,
  ...

};

Could also supply a function as an option that replaces keys for a given file.

[Rich-Harris]

Also, the above should only apply if the file describes an object, and not an array, because that would be weird