Add discriminator for ICANN and PRIVATE sections

[peterthomassen]

The PSL is devided in several sections, currently ICANN and PRIVATE. https://publicsuffix.org/list/ explains:

While some applications, such as browsers when considering cookie-setting, treat all entries the same, other applications may wish to treat ICANN domains and PRIVATE domains differently. For example, Certification Authorities checking for wildcard misissuance would not issue a ".com" wildcard cert ("com" is in the ICANN domains list) but could legitimately issue a ".appspot.com" wildcard cert to the domain owner, in this case Google ("appspot.com" is in the PRIVATE domains list).

psl-dns currently does not expose this information to the user. In this issue, I'd like to collect reasons for and against adding some sort of discriminator.