Adds `--exclude-tags` option to filter out tasks

This commit implements --exclude-tags option of ansible-playbook command, which can be used to filter out tasks tagged with any of the given comma-separated tags.

```
# Execute tasks tagged as `hadoop`, but not as `hbase` or `zookeeper`
ansible-playbook -i hosts playbook.yml --tags=hadoop --exclude-tags=hbase,zookeeper
```

The implementation is pretty simple and I find this feature quite useful sometimes.
What do you think?