Auto paginate contributors so that everyone is listed on humans.txt

So I realized today that `humans.txt` isn't listing all the contributors of the project due to GitHub paginating results.

This requires a GitHub API token to work because there are 127 contributors at this point, and I have a feeling that number is only going to get bigger :smile: 

I'd like to add caching as well to this process, but I'm not entirely sure where it is appropriate to put it with out breaking other Octokit instances.
It's also worth considering that calling for *all* contributors could at some point take more than 5 seconds as this will continue to grow (hopefully). :grinning: