Add a way to configure ignore patterns per-source or per-store

[mdaeron]

It would be useful to define ignore files in some sort of config file, perhaps in sources.yml. This could look something like:

sources:
    toto:
        location: /Users/toto/mybackups
        ignore-file: /Users/toto/.ignore.txt

Alternatively, specify patterns instead of a file:

sources:
    toto:
        location: /Users/toto/mybackups
        ignore:
            - *.tmp
            - __pycache__
            - **/.cache

Not sure if this belongs in sources.yml or stores.yml. May depend on use case.