create an atexit trap for next_section

[scott-vsi]

git_mirror:next_section should have an ERR and/or EXIT trap setup so that when the program exits, the user's prompt doesn't inherit the color. This would avoid us having to call this at the end of each function that uses next_section in git_mirror:

  # Reset the color
  echo -n $'\e[0m'

We could also move it to colors.bsh. Cf. colors.bsh:get_color