added topleveldomain guesser

added a topleveldomain guesser to map tlds to a locale.
f.e:
domain.ch will map to locale de_CH

this is usefull for sites with more than one tld to provide a default locale per tld.

mappings can be influenced via config.
there is a subdomain guesser, which expects `{locale}.domain.com`. i guess it still has its use, as it avoids hardcoding a domain name at all.

but your guesser is not restricted to the top level domain, right? it is just an explicit map of domain names to locales. so i would like to challenge the name. what about simply `DomainGuesser`? or `DomainMapGuesser`?