Allow symbolic directory links in `static`

Feature requests
1

As a follow up of hard_link_static = true which is a huge improvement for my setup. Thank you.
It allows me to publish a whole site without copying any content:

  1. Assemble the static content with symbolic links to selected directories in you project.
  2. Reconstruct the tree with hard links to files as leaves in static.

For the latter I use the attached script.
Here my suggestion: include the scripts functionality in Zola.

#!/bin/sh
# FROM_DIR must be in the same directory where `config.toml` is.
FROM_DIR="blog3/static-symlinks"

cd "$FROM_DIR"
# create directories
find -L -type d -not -name . -exec mkdir -p "../static/{}" \;
# populate them with hard links
find -L -type f -not -name . -exec ln -L "{}" "../static/{}" \;