Add command line arg to disable loading the brave rewards extension

[mkarolin]

Description

Allow to provide command line arguments to npm start to disable loading Brave Rewards extension. This would facilitate debugging of the Brave Rewards extension by letting developers load the extension manually in dev mode.

CC @petemill @bsclifton @yrliou

Steps to Reproduce

1. npm start -- --disable_brave_rewards_extension

Expected result:

When --disable_brave_rewards_extension is passed the extension is not shown in the Brave Actions Bar