Query filters

A feature suggested by @icio that could improve performance for certain implementations. This pull request introduces the ability to specify accept/reject filters on the command line to force the namesever to behave in a certain way.

A great example is the ability to filter out IPv6 queries for certain domains, when it is known no `AAAA` records will exist for subdomains within. This can be a useful performance booster in some cases, as it removes the need to perform database lookups constantly for records that are guaranteed to not exist.

Here's an example invocation of discodns with these options used. The following nameserver will only ever respond successfully (not `NXDOMAIN`) to queries within the `discodns.net` domain, and of type `A`, `AAAA`, or `SOA`.

```
$ bin/discodns --accept="discodns.net:A,AAAA,SOA"
```

These can be combined together, and nested within each other, for example...

```
$ bin/discodns --accept="discodns.net:A,AAAA,SOA" --reject="foo.discodns.net:AAAA`
```

The above example will respond to any query for `A`, `AAAA`, or `SOA` within the `discodns.net` domain, but more specifically, will reject queries for `AAAA` inside the `foo.discodns.net` domain.

It may be worth noting the logic implementation does have limitations. For example, the following would not yield the desired result.

```
$ bin/discodns --reject="discodns.net:PTR" --accept="foo.discodns.net:PTR"
```

Desired: Reject any `PTR` queries within the `discodns.net` domain, *except* accept when it is within `foo.discodns.net`.

Result: Reject all `PTR` queries within the `discodns.net` domain.