Is the steem-plus extension open-sourced, because I couldn't seem to find it on github (though it's possible I missed it)?

Just as an FYI, there are a few other scripts and extensions that have been released by others (including myself) with similar functionality. My scripts (released a few months back), for example, are open-source TamperMonkey scripts that don't require you to again share your private keys:

Link: Ultimate STEEMIT: Minnow Post Vote Slider and Past Payout Monetizer Script... and next, here comes SteemTUBE!!! 😲

Link: "Hide ReSteems": TamperMonkey Script Toggles Feed & Profile ReSteems, now for GOLOS and STEEMIT!

For those who might not be as comfortable using a Chrome extension with STEEMIT (especially if it's not open-source), this might be another option to consider.