Multiple records

This PR adds support for multiple records.
There are cases where you can't just just have CNAMEs point to a single A which is the dynamic records. These include SRV and MX records which do not accept a CNAME as the target. So in these case, you must have multiple A records point to the same IP.

This PR also adds a user-agent header for providers which filter on this criteria.

Oh and it also logs on when actually modifying the IP (log level INFO).
We'll want to error if both are present in the config, in order to prevent user confusion. We'll also need to add something in the README to indicate that this is possible.