Allow DB location to be configured

[seeruk]

This would allow people to place their DB in things like Dropbox, allowing them to sync tid data across machines. It has the advantage that the sync will be handled in the background, and data will be eventually consistent without slowing down tid itself.

Originally, tidd was planned to do something similar (it would be a remote storage backend) but it could've introduced some latency, slowing down tid, to make it work the same as the above solution, it would need both a remote, and local daemon to handle the eventually consistent part, keeping tid fast. This is of course much more complex, we can just rely on Dropbox or w/e instead.