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).
I debated whether I should do this when originally implementing the tool, and I decided against doing so in order to comply with the intent of any webmasters that don't want scraping for their domain.

Since there are plenty of sites that don't seem to mind one bit about the user agent, let's go ahead and leave this out.