Improving PackRat

Although our PackRat implementation is functional, it is a bit limited, in that we can specify only one backup configuration at a time. If we want to use two or three backup directories, or if we want to back up some files on a different schedule than other files, we are out of luck with our current implementation. Another problem is that PackRat is a bit messy: It relies on the interval, to, and backup top-level methods.

A way around this is to redo the syntax for our packrat.pr file so that the user is actually creating and configuring multiple instances of Backup: