Add some initial unit tests

[JohnVillalovos]

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[YasharF]

I cherry-picked the first two commits into the currently maintained fork at https://github.com/SeattleDevs/letsencrypt-cloudflare-hook

Regarding the last three commits which involve changes in hook.py, I didn't have a chance to go through them, but is there any backward compatibility issues? In other words, do the current users need to change any of their config or scripts after merging those three commits?