Configurable TLDs

This pull request adds the ability to set a different TLD for a particular mapping. I was attempting to integrate OAuth2 with a third-party service, but they wouldn't let me register an app with a `.dev` TLD. This is my workaround so that I can have sites served locally from a `.com` TLD.

I personally like being able to set this per-mapping, as opposed to changing the TLD for all domains mapped with local-tld, but I could see adding a configuration option to set the global TLD as well. But that's for another time.