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.

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.
:thumbsup:

Perhaps it's worth doing the following rules...

```
--accept=".:A,AAAA" # Accept every domain, but only A/AAAA records
--accept="foo.net" # Accept only foo.net, but all record types
```