Security: Store API Keys in Secrets?

Hey fellas,

I just discovered your repository and I really think your implementation is beautiful.

But i just wondered if the API keys in [appveyor.uml](https://github.com/commandlineparser/commandline/blob/master/appveyor.yml) (Line 49,58) shouldn't be stored in secrets ?

Inspect appveyor.yml at lines 49 and 58, then review how this repository's AppVeyor configuration handles credentials. Done means the API keys are no longer stored directly in the configuration and the CI setup still has the required secret references; no test file is mentioned in the issue.

Written by the indexing model from the issue text.