Support --color option to indicate whether to use colors in the terminal

Mimic the behavior of the `--color=<auto|always|never>` found in tools like `ls` and `grep`. Default to `auto` which checks whether stderr is a tty to define whether colors are used.

Tested:
- Ran `./boostrap-vz --color=never` and `--color=always`, confirmed colorization worked as expected.
- Ran `./boostrap-vz --color=auto ${manifest_file} 2>bootstrap.log`, confirmed colors were not added to boostrap.log by default, repeated the test with `--color=always` and confirmed escape sequences were output.

Signed-off-by: Filipe Brandenburger <filbranden@google.com>