Hooks before/after a backup run

I need this feature so I created a clean branch with the relevant commits from https://github.com/meskyanichi/backup/pull/342. I also fixed the specs (logging calls were outdated). I thought it best to create a new pull request.

My use case is that I want to backup the state of my computer, including the list of installed applications and packages. I store these lists in temporary files with `ls` and other commands before the backup, and include them in my archives.