Mozilla::PublicSuffix provides a single function that returns the public suffix
of a domain name by referencing a parsed copy of Mozilla's Public Suffix List.
From the official website at http://publicsuffix.org:

A "public suffix" is one under which Internet users can directly register names.
Some examples of public suffixes are .com, .co.uk and pvt.k12.wy.us. The Public
Suffix List is a list of all known public suffixes.

A copy of the official list is bundled with the distribution. As the official
list continues to be updated, the bundled copy will inevitably fall out of date.
Therefore, if the bundled copy of found to be over thirty days old, this
distribution's installer provides the option to check for a new version of the
list and download/use it if one is found.