Support multiple CloudFlare accounts

[pjeby]

This PR implements support for multiple CloudFlare accounts, by trying credentials in order until an account is found that matches the current domain. Documentation is included in the patch, but basically you can add multiple, space-separated emails and api keys in the environment variables. The change is backward-compatible with existing environment variables. I tried to keep the code changes to the absolute minimum; hope you find it helpful.

[YasharF]

Since this repo is no longer maintained by its owner, now there is a new maintained fork at https://github.com/SeattleDevs/letsencrypt-cloudflare-hook. I tried cherry-picking your PR, and manual test to see if it is a breaking change was fine. However, it looks like the unit tests need to be modified in the other repo for this since the tox build test failed. Would you mind opening your PR against that repo and addressing the build issue as part of your PR? I would be happy to merge the change shortly afterward.

[pjeby]

It looks like the issue is that this PR changes the call signature of _get_txt_record_id() and return signature of _get_zone_id, but the new unit tests in that repo are dependent on the old signatures (because apparently they're testing private functions?).

[YasharF]

I took the unit tests that were in another PR since there were no tests, to begin with, without going through them much since they passed out of the box. I would be happy to accept PRs to modify the tests.