Allow user defined config file for dircolors

This was inspired by /etc/skel/.bashrc from Ubuntu 12.10. For details
about dircolors configuration file see dir_colors(5).
Global substitution when doing Git prompt modification

If prompt contains terminal title setting, then it is without it only
title would contain git related information.
Removing unnecessary export calls
Removing dead code left by Msys Git PS1 modification
Set terminal title using prompt for xterm and rxvt
Global substitution when doing Screen prompt modification

If prompt contains terminal title setting, then it is without it
only title would contain screen window number.