Tests always pass

Nobody has claimed this yet.

Please correct me if I'm wrong, but it seems the test cases provided will never fail.
This is because you're calling the `done()` callback with any errors, thus suppressing a possible failed test case.

For example, the following test passes:
```
it('should preserve order with multiple selectors', function(done){
Metalsmith('test/fixture')
.use(markdown())
.use(headings({ selectors: ['h1', 'h2']}))
.build(function(err, files){
if (err) return done(err);
expect(1).toEqual(2)
done();
});
});
```

I'd suggest just letting any error through.

Start with the test cases that use the asynchronous build callback, including the example for preserving order with multiple selectors. Run the test suite and verify that an intentionally failing assertion is reported rather than hidden by the callback handling. Done means test failures propagate correctly while successful asynchronous tests still pass.

Written by the indexing model from the issue text.