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>
Check the value of the --color argument

Make sure it's either `auto' (the default), `always' or `never'. If it
does not match any of the values, raise a Docopt exception that causes
it to print usage and exit.

Tested:
- $ sudo ./bootstrap-vz --color=invalid manifests/gce.manifest.yml
  Value of --color must be one of auto, always or never.
  Usage: bootstrap-vz [options] MANIFEST

Signed-off-by: Filipe Brandenburger <filbranden@google.com>
Merge pull request #143 from filbranden/color_when

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