Colored virtualenv name in prompt ($PS1)

  - A more visual way to know if the virtual environment is currently
  activated.
  - The virtual environment name at the beginning of the prompt ($PS1) will
  appear in color.
  - By default the color green is used. This can be changed using
  'export VIRTUAL_ENV_PS1_COLOR=$COLOR_CODE' before sourcing activate script.
  - For bash only.
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>