Provide `--username` and `--password` cli args

[hstenzel]

Currently, helm upgrade supports --repo, --username, and --password

$ helm upgrade --help | grep -e '--username ' -e '--password ' -e '--repo '
      --password string              chart repository password where to locate the requested chart
      --repo string                  chart repository url where to locate the requested chart
      --username string              chart repository username where to locate the requested chart

However, helm diff upgrade only supports --repo

$ helm diff upgrade --help | grep -e '--username ' -e '--password ' -e '--repo '
      --repo string                  specify the chart repository url to locate the requested chart

I'd like to be able to pass in the the credentials to the helm repo when using the --repo arg. While it's currently possible to achieve the same result by helm repo adding the the repo with user then changing the chart, this introduces config changes on the local system and causes the command usage to diverge.

Thanks in advance! Great project!