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:

documentation/__tests__/bin.js

Lines 371 to 378 in a31e732

	test.skip('build --document-exported', async function () {
	const data = await documentation(
	['build fixture/document-exported.input.js --document-exported -f md'],
	{},
	false
	);
	expect(data).toMatchSnapshot();
	});

[chriskrycho]

@buschtoens mind rebasing this? Happy to land it!