Need commonjs export patch for build

Babel 6.x do not transform `export default` to `module.exports`, and most babel presets are written in commonjs which do not compatible with `exports.default`.

Solution:
Change `"build": "babel src -d lib"` to `"build": "babel src -d lib && echo \"\nmodule.exports = exports['default'];\" >> lib/index.js"`

Start in package.json and inspect the build script that runs Babel from src to lib. Run the build and check lib/index.js; done means the generated entry point exposes the default export through CommonJS as described.

Written by the indexing model from the issue text.