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)
Copy content before passing it to Kramdown

Kramdown 1.0.0 calls #encode! on the source string, but the one passed
by nanoc is frozen.
Fix forwardable methods in 1.8.x
Merge pull request #278 from bobthecow/vintage-forwardable

Fix forwardable methods in 1.8.x
Merge pull request #276 from cdlm/fix-kramdown-v1

Copy content before passing it to kramdown, for compatibility with 1.0.0