Ability to define a custom prompt pwd

Based on #88, I created a way to customize the prompt pwd info. Example:

```sh
export PURE_PROMPT_PWD=$(echo $PWD | sed -e "s|^$HOME|~|" -e 's|^/private||' -e 's-\([^/.]\)[^/]*/-\1/-g')
source ~/pure.zsh
```

![screen shot 2014-10-05 at 3 36 11 pm](https://cloud.githubusercontent.com/assets/245435/4519491/823736d8-4cbe-11e4-8f27-0b7e28c1543d.png)

What do you think?