Before:
$ GH_ENTERPRISE_TOKEN="..." gh pr create
could not find hosts config: not found
Now:
$ GH_ENTERPRISE_TOKEN="..." gh pr create
set the GH_HOST environment variable to specify which GitHub host to use
Also amends `gh help environment` documentation to suggest the use of
GH_HOST when scripting operations with GitHub Enterprise repositories.
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.
