Support for multiple TLDs

[jonaharagon]

The configuration option should allow you to specify a list of TLDs the program will accept. For example (in config.json), a list like this would be great.

{
"tlds": [ "tld", "o", "free" ]
}

And et ceteta. They would be the "acceptable" top-level domains for registrations (and domains would go in the correct zones accordingly during registration).

This would also make it better if some other operator were to use Cesium, as our domains wouldn't be hard-coded in.