title: Using boto3 to add extra tags to S3 buckets
body: <p>Currently I'm in the process of tagging every S3 bucket I have using boto3. Compared to a resource like Lambdas, doing <code>s3.put_bucket_tagging</code> overwrites any previous tags, compared to Lambdas which only add extra tags while keeping old ones. Is there a way to only add tags, rather than overwrite them?</p>
  
  <p>Secondly, I have created a method to take the current tags, add the new tags on, and then overwrite the tags with those values, so I don't lose any tags. But some of these S3 buckets are created by CloudFormation and thus are prefixed with <code>aws:</code> which gives me the error <code>Your TagKey cannot be prefixed with aws:</code> when I try to take the old tags and re-put them with the new tags. </p>
  
  <p>A fix for either of these to give me the ability to automate tagging of every s3 bucket would be the best solution.</p>