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).
Instead of using a comma-delimted list, let's go with an array. In this case, that would be:

```json
  "names": ["@", "mail", "xmpp"]
```

If we've got JSON at our disposal, let's use it!