PURE_PROMPT_SYMBOL added

I use the ```Terminus``` font (v4.39) and the output looks like this one:
![2014-12-28-82dac47](https://cloud.githubusercontent.com/assets/305739/5563996/7380c0b6-8ea2-11e4-8bf5-41c816105838.png)

For a long time I've used the update script like this:
```sh
wget -O pure.zsh-theme.new \
  "https://raw.githubusercontent.com/sindresorhus/pure/master/pure.zsh" && \
  sed 's/❯%f /￫%f /' pure.zsh-theme.new > pure.zsh-theme && \
  unlink pure.zsh-theme.new
```
Right now it is possible to change the delimiter via variable.
![2014-12-28-29212f7](https://cloud.githubusercontent.com/assets/305739/5563998/79de52a2-8ea2-11e4-9fd4-effed13479ac.png)

Please, review. Thank you.
converts the output to null if PURE_PROMPT_SYMBOL is not set