Create a central backup folder

[astorm]

Instead of backing up files in place, let's add the ability to create a central backup folder somewhere on the user's computer, and have the files backed up there instead. Top of head, this would mean

1. Adding a new configuration item to the ~/.pestle/ folder (to hold the backup folder)
2. Adding a new command to configure a backup folder
3. Where should we backup by default? In place? How do we let users know this is an option?
4. Finding every place we create a .bak file in the source, getting them to use a single function for this backup, and then having that function reference the ~/.pestle/ config item.