Add way to disable color prompt at venv creation time

When the virtual environment is created - by default - will create a color
prompt. Add way to disable color prompt by setting VIRTUAL_ENV_DISABLE_COLOR
environment variable before creating the venv.

Usage:

# create no-color prompt
$ export VIRTUAL_ENV_DISABLE_COLOR=1
$ virtualenv venv

Signed-off-by: Nicolás Pernas Maradei <nicopernas@gmail.com>
Document color prompt feature

Signed-off-by: Nicolás Pernas Maradei <nicopernas@gmail.com>