ValidTLD option added

A new default option has been introduced in DomainClassifier which
is the validTLD option (enable by default). Based on the assigned
list of TLD, the extraction of potential domains is filtered to the
IANA assigned list.

If you are extracting the data for non-assigned/internal TLDs, you can disable
the default option with  validTLD=False on the potentialdomain function.

The list of assigned TLDs is downloaded from IANA.