fix: handle default export. fixes #7

[shellscape]

This PR contains:

• [x] bugfix
• [ ] feature
• [ ] refactor
• [x] tests
• [ ] documentation
• [ ] metadata

Breaking Changes?

• [ ] yes
• [x] no

If yes, please describe the breakage.

Please Describe Your Changes

Issue #7 raised a problem with ES6 module configs that exported a default. The default export wasn't recognized and the entire export was passed to webpack, resulting in a validation error.