Make prohibited_domain_characters_regex changeable
Make prohibited_domain_characters_regex changeable

On some SMTP servers you are not allowed to send mails to UTF-8 addresses. On my side its not allowed
to have "umlaut" in domain part.

This patch allows you to change the prohibited_domain_characters_regex such as a initalizer:

```ruby

ValidEmail2::Address.prohibited_domain_characters_regex = /[+!_\/\s'`äöüÖÄÜß]/
```

lisinge/valid_email2