Refactored out color defs & assignments; other optimizations.

I factored out all color definitions into separate files. `gitprompt.sh` now sources files for color names and git prompt color assignments.  I had to make the prompt color names global (but not exported) because of the function nesting levels.

These files can reside as a dot-file in ~, or the normally named file in ~/lib, or in the directory from which the `gitprompt.sh` script is running.

The git status processing was also refactored.

I added a space after a few of the flag characters to improve readability; the flag characters leave almost no space between itself and the subsequent number.

*TODO*: _Need some kind of regression test_.