It's better to set this per-host in general, since setting the top-level
`git_protocol` key will not have any effect if the per-host setting
exists, and it does by default for anyone who ran `auth login`.
Adding GITHUB_TOKEN & GITHUB_ENTERPRISE_TOKEN support orthogonal to
Config was getting out of hand, especially in `auth` commands that
adjust their messaging and error status based on the presence of these
environment variables.
The new approach builds in support for tokens from environment straight
into Config object by composition. Thus, commands need not ever be
concerned with any specific environment variables.
