Replace github-api package

[kb0rg]

The npm github-api package we currently use is facing a planned deprecation. While the timeline for this is not pressing, it might behoove us to get ahead of it.

They recommend moving to Octokit, though it's not yet published on npm

Note: This issue may be redundant to #52 -- but it's possible they both need to be done, just within different timeframes.