Tag release prefix

This PR allows to pass a custom tag prefix to the `scc tag-release` command. In terms of use case, it should allow us to create tags with non-default prefixes for demo tagging for instance using:

```
scc tag-release RELEASE --prefix demo/
```

Integration tests are added to `test_tag_release.py` to check this new behavior. To test this PR,
- check the command above works with different prefixes and does not change the behavior is no prefix is passed (i.e. use the prefix of the last tag)
- check the `test_tag_release` integration tests are working correctly.