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).
Additionally, in order to support users of the existing tool, we're going to have to be backwards-compatible with the existing way of configuration.

This means we'll still need to support a `"name"` field that has a single string in the config.