Support the export default syntax

[7studio]

Hi,

before opening this pull request I read closely the issue #7.
It's a bit annoying that the syntax export default was the only one not supported by the plugin. troygoode/node-require-directory#31 seems to have been widely forgotten for over a year now and we could use this "patch" for now.

I added more code in /test directory than into the index.js file but I hope you appreciate these changes.

[slavafomin]

Hello! Thank you for the PR. My time is extremely limited these days, which explains why I hadn't a chance to properly maintain this project. However, I will definitely try to look into this.

[slavafomin]

I'm so sorry, but this extension is deprecated and archived now. You can still use it in legacy projects with Gulp 3, but I'm not planning to fix the issues or add new features, as well as merging any PRs.

🚀 Please, consider migrating to Gulp 4 with Gulp Hub instead.

❤️ Thank you for using this library in the past. I wish you the best of luck with your projects!