Hiawatha comes with a script to acquire and maintain Let's Encrypt certificates. It's included in the Hiawatha source package (directory extra/letsencrypt), but you can also download it as a separate package via the links below.
Usage: ./letsencrypt <command> Commands: register: Register your account key at the Let's Encrypt CA. request <hostname> [<cert.pem>]: Request new certificate for website. expire: show number of days left before certificate expires. renew [restart]: Renew the almost expired Let's Encrypt certificates in Hiawatha's certificate directory. revoke <cert.pem>: Revoke the certificate.