Throttling mechanism to manage content creation abuse rate limit

[scottwn]

Alternate approach to #109, using plugin-throttling minimumAbuseRetryAfter instead of sleep.

I used this discussion to guide this solution.

Fixes #28

[spruce]

This should be resolved with merging of #109 , right? If so, please close this PR. :)

[scottwn]

I think this PR still has some worthwhile improvements:

• Log message that communicates the length of the retry.
• By using minimumAbuseRetryAfter instead of sleep, it allows octokit to handle the retryAfter header if it's returned by the GitHub API. Crash with abuse rate limits #28 was caused by that header not being returned in certain cases, which is why we need to add the extra delay. But in cases where the header is returned, we don't want to add more sleep on top of that.

[spruce]

@scottwn I'd like to merge it. Could you resolve the conflicts for that?