Feature/DNS audit

Separated audit features for verifier host.

Truemail.host_audit

=> #<Truemail::Auditor:0x00005580df358828
@result=
  #<struct Truemail::Auditor::Result
    current_host_ip="127.0.0.1",
    warnings={
      :dns=>"a record of verifier domain not refers to current host ip address",
      :ptr=>"ptr record does not reference to current verifier domain"
    },
    configuration=
    #<Truemail::Configuration:0x00005615e86327a8
      @blacklisted_domains=[],
      @connection_attempts=2,
      @connection_timeout=2,
      @default_validation_type=:smtp,
      @email_pattern=/(?=\A.{6,255}\z)(\A([\p{L}0-9]+[\w|\-|\.|\+]*)@((?i-mx:[\p{L}0-9]+([\-\.]{1}[\p{L}0-9]+)*\.[\p{L}]{2,63}))\z)/,
      @response_timeout=2,
      @smtp_error_body_pattern=/(?=.*550)(?=.*(user|account|customer|mailbox)).*/i,
      @not_rfc_mx_lookup_flow=false,
      @smtp_safe_check=false,
      @validation_type_by_domain={},
      @verifier_domain="example.com",
      @verifier_email="[email protected]",
      @whitelist_validation=false,
      @whitelisted_domains=[]>

1. Implement separated DNS audit for verifier host
2. Refactored Truemail::Auditor, Truemail::Audit::Base, Truemail::Auditor::Result
3. Extracted IP determining logic to independet auditor class
4. Refactored PTR audit logic
5. Updated gem documentation
6. Updated gem version to 1.8.0