allow other backends for storing duplicate documents

[mattilyra]

at the moment all the documents are stored in an in-memory database - it should be possible to define this to be anything that supports getting/setting items

[RiseljeVoliMacke]

Hey, @mattilyra, I see that there's the issue-4-backends branch and SqliteBackend implemented there - but that is not yet completed, is that right?

[devtanna]

Hello, @mattilyra ; I too would love to try this implementation but would need a persistent backend. Something like redis or MySQL.; When I have the time I can give it a try :)