fix(exported): respect `parse-extension` & `require-extension`

[buschtoens]

Fixes #1272. Fixes #1258.

I was gonna add a test based on the existing --document-exported case, but unfortunately it's skipped: https://github.com/documentationjs/documentation/blob/a31e73219c553c1a1e6c49e9094293f2c2c4608f/tests/bin.js#L371-L378