Add SPF support for RecordConfig (#1020)

This type is identical to TXT but used for other purposes, it is
officially supported by OctoDNS.

Co-authored-by: Tom Limoncelli <tlimoncelli@stackoverflow.com>

models/target.go

@@ -23,7 +23,7 @@ so that it is easy to do things the right way in preparation.
 // GetTargetField returns the target. There may be other fields (for example
 // an MX record also has a .MxPreference field.
 func (rc *RecordConfig) GetTargetField() string {
-	if rc.Type == "TXT" {
+	if rc.HasFormatIdenticalToTXT() {
 		return strings.Join(rc.TxtStrings, "")
 	}
 	return rc.Target