fix: default type for the config file option

Otherwise this happens:

```
core.py 2151 __init__
raise ValueError(

ValueError:
'default' must be a list when 'multiple' is true.
```

#60

Author: danihodovic

src/cli.py

@@ -17,7 +17,7 @@ def config_opt(func):
     return click.option(
         "-c",
         "--config",
-        default="/etc/borgmatic/config.yaml",
+        default=["/etc/borgmatic/config.yaml"],
         help="The path to the borgmatic config file",
         multiple=True,
         type=click.Path(