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_mx.go

@@ -0,0 +1,40 @@
+package models
+
+import (
+	"strconv"
+	"strings"
+
+	"github.com/pkg/errors"
+)
+
+// SetTargetMX sets the MX fields.
+func (rc *RecordConfig) SetTargetMX(pref uint16, target string) error {
+	rc.MxPreference = pref
+	rc.Target = target
+	if rc.Type == "" {
+		rc.Type = "MX"
+	}
+	if rc.Type != "MX" {
+		panic("assertion failed: SetTargetMX called when .Type is not MX")
+	}
+
+	return nil
+}
+
+// SetTargetMXStrings is like SetTargetMX but accepts strings.
+func (rc *RecordConfig) SetTargetMXStrings(pref, target string) error {
+	u64pref, err := strconv.ParseUint(pref, 10, 16)
+	if err != nil {
+		return errors.Wrap(err, "can't parse MX data")
+	}
+	return rc.SetTargetMX(uint16(u64pref), target)
+}
+
+// SetTargetMXString is like SetTargetMX but accepts one big string.
+func (rc *RecordConfig) SetTargetMXString(s string) error {
+	part := strings.Fields(s)
+	if len(part) != 2 {
+		return errors.Errorf("MX value does not contain 2 fields: (%#v)", s)
+	}
+	return rc.SetTargetMXStrings(part[0], part[1])
+}