Coloured 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.