Add an option to deactivate transfer for specific file types

[Joacchim]

When migrating data from one backend to another, where the feature support is different between the two, we would like to be able to tell cloudmig not to try and migrate some specific kind of files. For instance, the S3 backend does not support symlinks, and as such, we would like to prevent migrating any symlink during a migration to this backend, suppressing errors from symlink migrations.