Add "last updated" mode

[lippoliv]

Currently we support "last added" mode, so we remember the ID of the last transfered document and just transfer, what matches the criteria + has a higher ID. But human may be edit documents and make them "later" match the criteria. In this case, this tool wouldn't help.

So we should add a new mode "last update" which would upload every changed file until we reach the last uploaded document. Users must be able to pick one of those two modes since both have drawbacks (last updated could end up in duplicates).