v1.0.0: Feature/Configurable record not found behaviour

Added configurable strategy for record not found case. By default it won't raise an exception when DNS record not found in mocked records dictionary:

DnsMock.start_server(port: 5300)

dig @localhost -p 5300 A example.com

; <<>> DiG 9.10.6 <<>> @localhost -p 5300 A example.com
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 38632
;; flags: rd; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 1
;; WARNING: recursion requested but not available

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;example.com.			IN	A

;; Query time: 0 msec
;; SERVER: 127.0.0.1#5300(127.0.0.1)
;; WHEN: Fri Jan 29 08:21:30 EET 2021
;; MSG SIZE  rcvd: 40

If you want raise an exception when record not found, just start DnsMock with exception_if_not_found: true option:

DnsMock.start_server(exception_if_not_found: true)

1. Updated DnsMock.start_server
2. Updated DnsMock::Server
3. Updated DnsMock::Response::Message
4. Updated DnsMock::Response::Answer
5. Updated readme, changelog
6. Updated gem version to 1.0.0