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{...}"`.