Warn when nearing unauthenticated API limits

[ogham]

GitHub performs rate limiting using HTTP headers:

X-RateLimit-Limit: 60
X-RateLimit-Remaining: 51
X-RateLimit-Reset: 1527514890

StackOverflow uses throttles:

{
    ...,
    "quota_max":300,
    "quota_remaining”:278
}

The easiest way around this is to authenticate, but as a command-line application, we can’t do that. So instead, checkback should display a warning when the number of requests is nearing its limit, instructing the user to wait.