Update start.command.ts to allowUnknownOption

Commander requires an explicit call to `allowUnknownOption` to allow and not error on options it does not recognize. This causes an issue with [this functionality](nestjs#1847) because any user-defined flags are blocked at the command level.

commands/start.command.ts

@@ -8,6 +8,7 @@ export class StartCommand extends AbstractCommand {
   public load(program: CommanderStatic): void {
     program
       .command('start [app]')
+      .allowUnknownOption()
       .option('-c, --config [path]', 'Path to nest-cli configuration file.')
       .option('-p, --path [path]', 'Path to tsconfig file.')
       .option('-w, --watch', 'Run in watch mode (live-reload).')