Added color configuration options

The following variables can now be used to control various aspects of
the prompt:

```
prompt_pure_dir_color="%F{magenta}"
prompt_pure_vcs_color="%F{cyan}"
prompt_pure_exec_time_color="%F{blue}"
prompt_pure_success_color="%F{cyan}"
prompt_pure_failure_color="%F{yellow}"
```

Note that all variables take a string of the same form as would normally
be used to colorize the prompt, i.e. one beginning with `"%F{...}"`.