Ability to define a custom prompt pwd

Based on #88, I created a way to customize the prompt pwd info. Example:

```sh
function my_custom_prompt_pwd() {
  echo $PWD | sed -e "s|^$HOME|~|" -e 's|^/private||' -e 's-\([^/.]\)[^/]*/-\1/-g'
}
export PURE_PROMPT_PWD=my_custom_prompt_pwd
source ~/pure.zsh
```

![screen shot 2014-10-05 at 3 48 10 pm](https://cloud.githubusercontent.com/assets/245435/4519538/2fd1fa16-4cc0-11e4-9a2e-1cf6d275888a.png)


What do you think?