This allows the same environment variables to be read, parsed, and used
from any API client as was previously handled in the CLI. The CLI now
uses the API environment variable reading capability, then overrides any
values from command line flags, if necessary.
Fixes#618
This strips out http.DefaultClient everywhere I could immediately find
it. Too many things use it and then modify it in incompatible ways.
Fixes#700, I believe.
Jeff Mitchell