fixed whitespace-stripping, replace all whitespace in domains.txt entries with single space character (fixes #60 and #78)

add support for dns-01 challenge - try 2

removed get_json_array helper, rearranged sign_domain method a bit, made grep for PEM-format-check quiet

rewritten command_revoke method (shortened + changed output to look less like one of those weird openssl error messages)

rewritten argument handling and moved to new main() method, added dependency checks, added _exiterr helper

Changed shebang of import-certs.sh to #!/usr/bin/env bash

Renew certificate 30 days before expiration

fix typo in test.sh (ngrok not found)

According to https://community.letsencrypt.org/t/quick-start-guide/1631
Renewals and Lifetimes:
Certificates from Let's Encrypt are valid for 90 days. We recommend
renewing them every 60 days to provide a nice margin of error.

when using printf, we do not need inline expansion of variables