Make enum values typed strings (#18)

This also moves validation into the parsing process and refactors a bunch of code related to the config.

Reviewed-on: #18
Co-authored-by: Anna Rose Wiggins <annabunches@gmail.com>
Co-committed-by: Anna Rose Wiggins <annabunches@gmail.com>

internal/configparser/devicetype.go

@@ -0,0 +1,40 @@
+package configparser
+
+import (
+	"fmt"
+	"strings"
+)
+
+type DeviceType string
+
+const (
+	DeviceTypeNone     DeviceType = ""
+	DeviceTypePhysical DeviceType = "physical"
+	DeviceTypeVirtual  DeviceType = "virtual"
+)
+
+var (
+	deviceTypeMap = map[string]DeviceType{
+		"physical": DeviceTypePhysical,
+		"virtual":  DeviceTypeVirtual,
+	}
+)
+
+func ParseDeviceType(in string) (DeviceType, error) {
+	deviceType, ok := deviceTypeMap[strings.ToLower(in)]
+	if !ok {
+		return DeviceTypeNone, fmt.Errorf("invalid rule type '%s'", in)
+	}
+	return deviceType, nil
+}
+
+func (rt *DeviceType) UnmarshalYAML(unmarshal func(data interface{}) error) error {
+	var raw string
+	err := unmarshal(&raw)
+	if err != nil {
+		return err
+	}
+
+	*rt, err = ParseDeviceType(raw)
+	return err
+}