Support creating parent directories as needed

[mcieno]

I oftentimes find myself needing a multilevel folder structure in the tmp folder (e.g., tmp/cmd1/..., tmp/cmd2/...).

Unfortunately, tmp_dir = tmp/cmd1 fails unless tmp/ already exists.
Although this is very easy to work around by creating the parent directories beforehand, I believe that Air should be able to recursively create parents by default.

This change is equivalent to running mkdir -p "$tmp_dir" instead of mkdir "$tmp_dir".