v1.2.0: Feature/Ability to specify MX record priority

Added ability to specify custom priority of MX record if it needed. Now it impossible to define null or backup MX records. Please note, if you haven't specified a priority of MX record, it will be assigned automatically. MX records builder is assigning priority with step 10 from first item of defined MX records array.

require 'dns_mock'

records = {
  'example.com' => {
    mx: %w[.:0 mx1.domain.com:10 mx2.domain.com:10 mx3.domain.com]
  }
}

DnsMock.start_server(records: records, port: 5300)

dig @localhost -p 5300 MX example.com

; <<>> DiG 9.10.6 <<>> @localhost -p 5300 MX example.com

;; ANSWER SECTION:
example.com.		1	IN	MX	0 .
example.com.		1	IN	MX	10 mx1.domain.com.
example.com.		1	IN	MX	10 mx2.domain.com.
example.com.		1	IN	MX	40 mx3.domain.com.

;; Query time: 0 msec
;; SERVER: 127.0.0.1#5300(127.0.0.1)
;; WHEN: Wed Feb 03 20:19:51 EET 2021
;; MSG SIZE  rcvd: 102

1. Updated DnsMock::Record::Builder::Mx
2. Updated readme, changelog
3. Updated gem version to 1.2.0