Do not move files out of `content/` (fixes #271)

This is not as efficient as it could be, because moving files out of the
temporary directory into `output/` is perfectly fine. Copying binary
items into the temporary directory just so they can be safely moved is
also not efficient either, though.

And yes, @bobthecow was right all along.
Merge pull request #272 from ddfreyne/bug-move-files-out-of-content

Do not move files out of `content/` (fixes #271)