Add command to prune empty directories

[svniemeijer]

This command line operation should remove any empty directories (recursively) from a product archive.

[bruno-rino]

What to name this tool? muninn-prune-filesystem?

[svniemeijer]

There currently exists a simple alternative which is to use find <archiveroot> -type d -empty -delete

The priority of this feature is thus low.
We should consider including it when we tackle #7.

[svniemeijer]

Since this feature is only applicable for storage=fs, it does not really fit as a muninn command.
The current recommended approach will thus be to use the find <archiveroot> -type d -empty -delete approach.