Move from multiple `--tag` flags to one single `--tags` flag

[benoit74]

Currently, when the user wants to specify multiple ZIM tags, he has to specify multiple time the --tag parameter like in sotoki --tag "Quality Assurance" --tag "Another TAG"

This kind of this is not supported by the Zimfarm and is not in line with other scrapers behavior.

We hence want to move to one single --tags flag which might contain a CSV value like in sotoki --tags "Quality Assurance,Another TAG". Commas are not expected to be used in tag values.

NOTA: this is a breaking API change ... but I'm not sure this is really widely used anyway, so I suggest to still make it into 2.2.0 (instead of creating a 3.0.0).