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.