Add logic to create tag while updating submodules

- Add logic to tag Git repositories
- Implement tagging logic as option in update-submodules

See openmicroscopy/snoopys-sandbox#125 as an example generated locally with
```
$ git checkout origin/dev_4_4 && git submodule update
$ python ~/code/scc/scc.py update-submodules --tag test --push tag dev_4_4
```

NB: not sure if merging openmicroscopy/snoopys-sandbox#125 through the GH interface would push the tag created in the submodule. But as discussed with @joshmoore, we may want to signoff and push tags manuallt when accepting releases anyways