handle existing releases idempotently?

i have a repo with a series of charts in it and i just wrote a script to iterate over each chart
calling `helm package` on every chart, and then call `cr upload` on the result.

works the first time, but then the second time i get 422's.

to account for this situation, do you think `cr` should ignore or warn on duplicates?

or do you think that the consumer should be smart enough to only generate packages for those things that have changed?

```
docker run \
--rm \
-v /Users/tony/git-alt/helm-charts/build:/build \
-e CR_OWNER=the-watchmen \
-e CR_GIT_REPO=helm-charts \
-e CR_PACKAGE_PATH=/build \
-e CR_TOKEN=$CR_TOKEN \
quay.io/helmpack/chart-releaser:v1.0.0-beta.1 \
cr upload
```
```
Error: error creating GitHub release: POST https://api.github.com/repos/the-watchmen/helm-charts/releases: 422 Validation Failed [{Resource:Release Field:tag_name Code:already_exists Message:}]
```