v2.2.0: Feature/SMTP fail fast

Ability to use fail fast behaviour for SMTP validation layer. When smtp_fail_fast = true it means that truemail ends smtp validation session after first attempt on the first mx server in any fail cases (network connection/timeout error, smtp validation error). This feature helps to reduce total time of SMTP validation session up to 1 second.

Truemail.configure do |config|
  config.smtp_fail_fast = true # by default it's equal to false
end

1. Added Truemail::Configuration#smtp_fail_fast
2. Added Truemail::Validate::Smtp#smtp_fail_fast?
3. Added Truemail::Validate::Smtp#filtered_mail_servers_by_fail_fast_scenario
4. Updated Truemail::Validate::Smtp#establish_smtp_connection
5. Updated readme, changelog
6. Updated gem version to 2.2.0