Merge target options with main options #701

If you have a build config like this...

// rollup.config.js
export default {
  entry: 'src/main.js',
  sourceMap: true,
  targets: [
    { 'dist/bundle.cjs.js', format: 'cjs' },
    { 'dist/bundle.es6.js', format: 'es6' }
  }
};

...both targets should inherit the sourceMap option. Don't think it makes sense to have to repeat shared options between targets – at the moment only the target object itself is passed to bundle.write(...).