add option to set locale based on tld of host domain

close #59
This PR adds a configuration option `tld_locales` allowing the `default_locale` to be set dynamically based on the TLD of the host domain. This is useful for apps hosted on several different domains where the native locale may be different in each case.

When the option is blank, the gem behaves exactly as before, with the option enabled it does behave a little differently though so it's more like an alternative use-case than an additional option.