Ensure target directory exists in `save!()`
Allow prefixing year/month for high volume archive

When you have a large number of screencaptures, it's useful to intelligently bucket them into manageable buckets by year and month.  Enabling "--date-dirs" appends this directory within the target directory.

This allows long-term archiving of a screenshot every 10s of our Aquarium without creating a crippling number of files.