backup: allow more than one -ignore-file

[mdaeron]

Is your feature request related to a problem? Please describe. From the users' perspective, it would make sense to separate different categories of ignore patterns. E.g., .DS_Store and ._* are likely to apply to all of my macOS backups, but depending on the specific sources I want to backup, I might have additional patterns to ignore.

Describe the solution you'd like It would be useful to be able to call something like:

plakar at @mybackups backup -ignore-file macos-ignore.txt -ignore-file src-specific-ignore.txt @src

Describe alternatives you've considered I could hack a script to concatenate ignore files on the fly, but who wants to live like that?