Helm CLI Fetch Artificat from Private GitHub Repo

Hi,

When using a private GitHub repository, the Helm client is not able to fetch the release artifacts due to the standard browser download url used in the generated `index.yaml`. It's mentioned in the following PR that the url in the `index.yaml` needs to be adjusted, and the PR also references a fix (https://github.com/helm/chart-releaser/pull/43) which introduced some changes.

PR: https://github.com/helm/chart-releaser/issues/14

I've checked the code, but it seems there is currently no way to tweak the actual download url in the generated `index.yaml` (e.g. like mentioned in this comment https://github.com/helm/chart-releaser/issues/14#issuecomment-577045042) or?

-- Sven