Add a backups config abstraction.

This abstraction helps address the problem of hard-coded paths for various sources from which backups pull data.  It also provides a better encapsulation of the information that drives backups.
This interface feels a bit strange to me. Why would we not just provide Backup with `files []string` and perhaps a function to call to perform a database dump?

Interfaces should be about behaviour. All I see here is function parameters.