Style overrides

Application of optional color over-rides are added on line 55. Any over-ride that is unset will be ignored. The naming scheme is PROMPT_*

Sorry if the multiple diffs on .bash_prompt are hard to follow in the aggregate. Some notes that might help:

- Because the default colors don't set the background color, I added several extra $resets to the PS1. After some testing I decided it was cleaner than setting the background color in the default palette.
- But since tput sr0 un-sets boldface, I added $bold to the definitions of each item of the default colorscheme, so that the default behavior will stay consistent with the master branch.
- Now, because I invoke $bold inside the default color scheme, I had to move the dimensioning of the "common color actions" up to line 17, so that they were already defined before we do anything else.
- It seemed most appropriate to have the last instance of $preposition_color moved into the git status sub-clause. This is arguably needed in the master branch (with or without style overrides), but I included it in this pull request because it caused a specific problem when using a tput setbg on PROMPT_PREPOSTION_COLOR. 
- Note that this might mean that get_prompt_symbol no longer has preposition_color applied to it. I liked it fine this way, but if you want, I can add another invocation of preposition_color to correct that. Another option worth considering would be a separate variable for prompt_symbol_color or somesuch, so that it could be configured (and overridden) separately.