Allow a config to update ALLOWED_TLDS

[kuahyeow]

Rather than updating the constant, and then waiting for a release, would it make sense to have a config option for gem users to configure the ALLOWED_TLDS ?

I guess users can update from http://data.iana.org/TLD/tlds-alpha-by-domain.txt, or customize as they require.

[kuahyeow]

Oh wait, there's already a valid_tlds option that is at the validator level. So maybe we don't need a global config after all.