Correctly create assets when output dir doesn't exist.

- Prior to this commit, if the output_dir did not exist then it
  would be nil for the entire build_docs, meaing the assets would fail
  to get a proper file handle to write to.
  (meaning, the first time you run hologram, it won't produce assets,
  just the asset directory).

Signed-off-by: Rajan Agaskar <rajan@pivotallabs.com>