Whenever a SSO challenge gets issued by the server by means of a URL in
the `X-GitHub-SSO` response header, gh now additionally prints that URL
on the standard error stream.
This is achieved by installing a middleware to all HTTP requests and
storing the server challenge to a value accessible by `factory.SSOURL()`.
Such approach was made necessary mainly because of the
`shurcool-graphql` client which doesn't give access to response headers
when a GraphQL error case is encountered.
