chromium_build.py: Rename to chromium.py
chromium.py: Enhance option parser

The following aspects are enhanced:
1. Replace optparse with argparse. It supports %(prog)s in epilog, has
built-in better format for epilog, and includes some other improvements
that might be useful for future.
2. Add support to run Chromium.
3. Group the options.
4. Display help when necessary.
.zshrc: Change name of Chromium script