curl_xml plugin: add AddressFamily

In situations when hostnames in URLs resolve to both IPv4 and IPv6
addresses, sometimes it's useful to have separate statistics for both
of these separately.

With this commit, within <URL> block you can set
  AddressFamily "ipv6"
or
  AddressFamily "ipv4"
to specifically use one or the other.

Signed-off-by: Jakub Jankowski <shasta@toxcorp.com>