Support for ICANN-only suffixes

[ghost]

The public suffix list contains both suffixes for ICANN domains and private suffixes. Using the flag only_icann the private suffixes can be deactivated:

>>> psl = PublicSuffixList()
>>> psl.publicsuffix("example.priv.at")
'priv.at'
>>> psl = PublicSuffixList(only_icann=True)
>>> psl.publicsuffix("example.priv.at")
'at'

[ko-zu]

I prefer not to ignore multiple icann sections as in official lint. Could you share your mind? https://github.com/ko-zu/psl/commit/a921595895ec6d19801a34afff6fac04129ae1fc

[ghost]

I didn't knew that there can be multiple ICANN sections, I'm fine with your the additional commit.

[ghost]

Thanks!