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 need to change this name in `config-example.json` as well, since that's the canonical example file we have.