Support persistent data storage

[joeldevlearning]

If using a backing file, users should have the option of saving their data and reloading on the next instance.

Persistence can just the basics and not involve UI changes:

• a flag must be set for persistence to be supported (i.e. don't overwrite a file)
• the program will check for filetype matching the type of storage; if not found, it will skip this step
• the storage type must NOT be in-memory; anything else

[joeldevlearning]

Hold off on this until new backends (#7) are added. Consider having persistent storage implement a new interface, IPersistantStorage, to complement the more basic IStorage.