PDF reports and unique databases

This version significantly changes the structure of stored data. No longer are all tasks stored in the same Deta database. Tasks are now stored in unique databases for their respective months, handled automatically based on the current time. In this way, the new export and previewmonth commands can be used to generate well-formatted PDF reports for any month during which tasks exist. See the export section of the README for sample reports and more information on using this function.