Add ability to replace a template

- This feature remains undocumented because it's not a good idea in most
  cases but it enables us to replace a template. This is especially
  useful when using sqlboiler as a library since testmain
  problematically loads config in it's own magical way, divorced from
  even the way sqlboiler in "normal" mode loads it. This enables
  replacement of that mechanism by replacing it's template.

main.go

@@ -79,6 +79,7 @@ func main() {
 	rootCmd.PersistentFlags().StringSliceP("blacklist", "b", nil, "Do not include these tables in your generated package")
 	rootCmd.PersistentFlags().StringSliceP("whitelist", "w", nil, "Only include these tables in your generated package")
 	rootCmd.PersistentFlags().StringSliceP("tag", "t", nil, "Struct tags to be included on your models in addition to json, yaml, toml")
+	rootCmd.PersistentFlags().StringSliceP("replace", "", nil, "Replace templates by directory: relpath/to_file.tpl:relpath/to_replacement.tpl")
 	rootCmd.PersistentFlags().BoolP("debug", "d", false, "Debug mode prints stack traces on error")
 	rootCmd.PersistentFlags().BoolP("no-tests", "", false, "Disable generated go test files")
 	rootCmd.PersistentFlags().BoolP("no-hooks", "", false, "Disable hooks feature for your models")
@@ -163,6 +164,14 @@ func preRun(cmd *cobra.Command, args []string) error {
 		}
 	}
 
+	cmdConfig.Replacements = viper.GetStringSlice("replace")
+	if len(cmdConfig.Replacements) == 1 && strings.ContainsRune(cmdConfig.Replacements[0], ',') {
+		cmdConfig.Replacements, err = cmd.PersistentFlags().GetStringSlice("replace")
+		if err != nil {
+			return err
+		}
+	}
+
 	if driverName == "postgres" {
 		cmdConfig.Postgres = boilingcore.PostgresConfig{
 			User:    viper.GetString("postgres.user"),