Override symlink dirs

This adds a `-s` flag, which is the opposite of `-S`: any argument, if it is a directory, is not symlinked but instead recurred down.

Closes #36.
In general I agree, and I'm going to make that change to this PR.

In the specific case of the exact test that you comment on: I disagree! There's a weird thing happening in that test that I want to preserve: it moves the directory, then symlinks the files, and the output reflects that.