Implements #23 - Ensure if servername config changes that certs are updated

[zsteinkamp]

Proposed changes

Recognize if njs_acme_server_names no longer matches the stored certificate. If so, request a new cert with the configured names.

Also includes a handful of small changes for DRYing up (e.g. adds acmeCommonName(r) and acmeAltNames(r) methods), defines some more specific types, renames some variables/methods to be more accurate. Adds a second server name to the example config to demonstrate this clearly.

These changes ended up making the PR bigger than I expected. Happy to remove the changes to make this one lean.

TODO: Add unit tests when #24 is merged (contains unit test framework improvements).

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING document
• If applicable, I have added tests that prove my fix is effective or that my feature works
• If applicable, I have checked that any relevant tests pass after adding my changes
• I have updated any relevant documentation (README.md and CHANGELOG.md)

[ivanitskiy]

LGTM