By now, ProFTPD used a default certificate with CN=*.snowflakehosting.ch on each server. There was no possibility to override the certificate with another one.
With this change, we added the options tls_key, tls_crt, tls_key_file, tls_crt_file which you can use according to the updated documentation.
Without setting on of those options, a self signed certificated bearing the hostname of the server will be created and used for ProFTPD by default.
Closes #302
