Rework the prompt config.

 * Move colors definitions in its own file.
 * Use prompt_subst for the path part, including git status when applicable.
 * Rework the battery status part of the prompt.
 * Rework the exit status.
 * Show the exit status only once, clear on empty line.
 * Rework git status, now clean of psvar
 * Now free of psvar.