Import/export of all shopping lists

[aperomsik]

OI Convert CSV is currently the recommended way to back up data from OI Shopping List. However as a backup solution it is somewhat inconvenient because it can only import and export one list at a time.

It would be useful to have a way for the user to specify that she wants to export all lists into a directory-- in files named as the list names -- and import all files in a directory into lists named after their file names.

[science]

+1