Domain filter middleware

*This is not ready to be merged yet (no tests at the moment), just wanted to go ahead and open it up for discussion.*

This adds a "domainFilter" middleware implementation, which allows you to easily extract bogart route style named parameters from the domain/host/authority portion of the URI. A potential use case is a multi-tenant application where the tenant is identified by a portion of the hostname. The relevant portions of the implementation are pretty similar to the bogart router.

TODO:

1. tests
1. support for multiple patterns