v0.2.0: Feature/PTR record support

Added ability to mock PTR records. Please note, you can define host address without RDNS lookup prefix (.in-addr.arpa). DnsMock will do it for you.

records = {
  '1.1.1.1' => {
    ptr: %w[domain_1.com domain_2.com]
  }
}

DnsMock.start_server(records: records)

dig @localhost -p 5300 -x 1.1.1.1

; <<>> DiG 9.10.6 <<>> @localhost -p 5300 -x 1.1.1.1
; (2 servers found)

;; ANSWER SECTION:
1.1.1.1.in-addr.arpa.	1	IN	PTR	domain_1.com.
1.1.1.1.in-addr.arpa.	1	IN	PTR	domain_2.com.

;; Query time: 0 msec
;; SERVER: 127.0.0.1#5300(127.0.0.1)
;; WHEN: Mon Jan 25 19:58:39 EET 2021
;; MSG SIZE  rcvd: 98

1. Added PTR record support
2. Updated gem version to 0.2.0