Added an SPF resource type.
Added an SPF resource type.

* Copied from
  https://github.com/agaridata/spf-ruby/blob/1f1f23e79885549bbdd53c6bf148c75c866b8434/lib/spf/eval.rb#L9-L12

thiago-sydow/spf-query,trailofbits/spf-query