Refactor RecordConfig: Add getters/setters (#314)

* Replace RecordConfig.Name and .NameFQDN with getters and setters.
* Replace RecordConfig.Target with getters and setters.
* Eliminate the CombinedTarget concept.
* Add RecordConfig.PopulateFromString to reduce code in all providers.
* encode and decode name.com txt records (#315)
* Replace fmt.Errorf with errors.Errorf

models/t_txt.go

@@ -0,0 +1,35 @@
+package models
+
+// SetTargetTXT sets the TXT fields when there is 1 string.
+func (rc *RecordConfig) SetTargetTXT(s string) error {
+	rc.Target = s
+	rc.TxtStrings = []string{s}
+	if rc.Type == "" {
+		rc.Type = "TXT"
+	}
+	if rc.Type != "TXT" {
+		panic("assertion failed: SetTargetTXT called when .Type is not TXT")
+	}
+	return nil
+}
+
+// SetTargetTXTs sets the TXT fields when there are many strings.
+func (rc *RecordConfig) SetTargetTXTs(s []string) error {
+	rc.Target = s[0]
+	rc.TxtStrings = s
+	if rc.Type == "" {
+		rc.Type = "TXT"
+	}
+	if rc.Type != "TXT" {
+		panic("assertion failed: SetTargetTXT called when .Type is not TXT")
+	}
+	return nil
+}
+
+// SetTargetTXTString is like SetTargetTXT but accepts one big string.
+// Ex: foo             << 1 string
+//     foo bar         << 1 string
+//     "foo" "bar"     << 2 strings
+func (rc *RecordConfig) SetTargetTXTString(s string) error {
+	return rc.SetTargetTXTs(ParseQuotedTxt(s))
+}