dynamo-browse: Re-add the clone command

[lmika]

Add the "clone" command back. This will work in a similar way to "new" — in that the partition/sort keys will be prompted for, and the created items will be marked as new — but it will start off as a duplicate of an existing item.

At the moment, only allow this to be usable for a single item. In the future, it may be possible to allow multiple items with a key-mapping option.