Moves cli/index.ts to async code
Moves cli/index.ts to async code

- Adds a comment to explain why process.exit(1) is used.
- Moves implementation to an async method instead of Promises and
  cleans-up the code.

GoogleChromeLabs/bubblewrap,GoogleChromeLabs/bubblewrap,GoogleChromeLabs/bubblewrap,GoogleChromeLabs/bubblewrap