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.
> I agree that it would be a strange choice, but it is more intuitive to make it so if I set PURE_PROMPT_SYMBOL to an empty char, that it respects that decision. Who are we to decide how others set up their prompt?

This is just like null/undefined in JS.

Sorry I don't see the use-case for an empty value and would much rather have it use the default in this case. The prompt symbol is there to make it clear that it's the prompt line and not output.