Add CLI arg for starting without Brave extension

Fix #150

Related: https://github.com/brave/brave-core/pull/87

You can do this with:

```
npm run start -- --disable_brave_extension
```

or

```
yarn start --disable_brave_extension
```

lib/start.js

@@ -13,6 +13,9 @@ const start = (buildConfig = config.defaultBuildConfig, options) => {
   if (options.no_sandbox) {
     braveArgs.push('--no-sandbox')
   }
+  if (options.disable_brave_extension) {
+    braveArgs.push('--disable-brave-extension')
+  }
 
   let cmdOptions = {
     stdio: 'inherit',

scripts/commands.js

@@ -51,6 +51,7 @@ program
   .option('--v [log_level]', 'set log level to [log_level]', parseInt, '0')
   .option('--user_data_dir_name [base_name]', 'set user data directory base name to [base_name]', 'brave-development')
   .option('--no_sandbox', 'disable the sandbox')
+  .option('--disable_brave_extension', 'disable loading the Brave extension')
   .arguments('[build_config]')
   .action(start)