Added a mechanism for filtering queries in the DNS server

If an administrator would like to limit the domains and/or types of records that can be queried through the nameserver, a set of "accept" or "reject" filters can be passed in to control this.

Reject: The query will be rejected if it matches any of the given filters
Accept: The query will be accepted if it matches any of the given filters
Include the ability to pass --accept and --reject through the command line
Refactored query filters into the `QueryFilterer`
Added basic unit test coverage for accept/reject filters
Added more tests (failing due to a bug!)
Added extra useful debugging messages
Changed the query filter check to match when no Accept filters are provided
Updated the readme for filters
Remove the production warning
Merge pull request #29 from duedil-ltd/feature/accept-reject-queries

Query filters