Added support for multi-tier domains (for example, .foo.bar)

* Also allow numerals in domain.

Note: this is backwards compatible (i.e. use of .dev still valid).

Our use case:

* Each developer has their domain (e.g. *.ryan.dev) - so we need to be able to have at least two tiers in the domain.
* We want to use a TLD with a numeral to avoid conflicting with potential gTLDs (.dev is currently being bid on).