add support for using custom whois host per domain

[serhatcetinkaya]

Description

intended as a fix for #92

Details

this doesn't break backwards compatibility. we can still continue using it with old config but with this patch we have the opportunity to specify whois host per domain in special cases, like:

domains:
- name: google.com
- name: facebook.com
- name: some.other.domain.with.special.whois.host
  host: whois.godaddy.com

[caarlos0]

I think we can probably do the breaking change and bump major...

or, we could do something like:

whois:
  - tlds: [ 'is', 'de' ]
    host: whois.godaddy.com

domains:
  - keep.as.is
  - google.de
  - foo.com

assuming my assumption that when you need to set a custom whois server, it usually is needed for the entire tld

[serhatcetinkaya]

@caarlos0 hello, that looks neat. but I suppose it will not work for all cases. for example let's say I have 2 domains with .es tld, one purchased from godaddy second from uniregistry. for one of them whois.godaddy.com will work, the other will get error and we might need to define a different host for it.

what do you think?

[caarlos0]

ah, that makes sense...