Allow disabling color with flag file (+script to turn off/on)

Did this after the last workshop:

  - some people are color-blind
  - Windows XP has a really bad terminal emulator with unreadable colors

This allows you to run `railsbridge-color off` from your VM, which will remove `~/.railsbridge_color`. Any additional prompt coloring we add from now on should test for the presence of this file.
How about doing `[ "$force_color_prompt" = "yes" ]` instead of checking for the file's existence again?