Allow users to explicitly define environment in conda.yaml / environment.yaml file

[adriangonz]

Let users manage their production-time dependencies explicitly through an optional conda.yaml file.

If the file is not present, it would then fall back into the current behaviour.

[jklaise]

I believe this is what I was talking about in #219, I assume this functionality now exists? Would be great to document so we can close #219 too.