Updates for Oauth2 support via Yagmail

[AndyHegemann]

I figured I would open this PR just incase anyone else ran into this annoyance with new google accounts.

While setting up this script, I made a new google account to send the notification emails from, but it turns out that new google accounts no longer let you enable "Less secure app access".

So I added configurable options to use yagmail and oauth2, which only requires the yagmail dependency if enabled. I also wanted the log file emailed to me if there was an error, so that got added as well. And, inline with #41 I added a check to abort if there are too many modified/updated files.