Allow setting options via a config file

Single-line configs via //go:generate were getting way too
long

Author: mromaszewicz

date_test.go

@@ -11,7 +11,7 @@ import (
 func TestDate_MarshalJSON(t *testing.T) {
 	testDate := time.Date(2019, 4, 1, 0, 0, 0, 0, time.UTC)
 	b := struct {
-		DateField Date	`json:"date"`
+		DateField Date `json:"date"`
 	}{
 		DateField: Date{testDate},
 	}
@@ -24,7 +24,7 @@ func TestDate_UnmarshalJSON(t *testing.T) {
 	testDate := time.Date(2019, 4, 1, 0, 0, 0, 0, time.UTC)
 	jsonStr := `{"date":"2019-04-01"}`
 	b := struct {
-		DateField Date	`json:"date"`
+		DateField Date `json:"date"`
 	}{}
 	err := json.Unmarshal([]byte(jsonStr), &b)
 	assert.NoError(t, err)