Add extra options hash for optional DNS options.

This allows for any additional DNS options that aren't directly exposed through the bind::server::conf class.  Looking to tune a caching-only server and there can only be one 'options' declaration, so the extra options aren't easily added via included files.

Example:

```
  bind::server::conf { '/etc/named.conf':
    acls              => {
      'trusted' => ['10.10.10.0/24', 'localhost'],
    },
    listen_on_addr    => ['any'],
    listen_on_v6_addr => ['any'],
    allow_query       => ['trusted'],
    allow_query_cache => ['trusted'],
    allow_recursion   => ['trusted'],
    forwarders        => ['10.10.0.11', '10.10.0.12'],
    extra_options     => {
      'recursive-clients' => '2000',
      'querylog'          => 'no',
      'lame-ttl'          => '300',
      'max-ncache-ttl'    => '60',
      'max-cache-ttl'     => '3600',
    },
  }
```