Make pure.zsh loadable by the prompt system.

With the code structured like this, pure can be loaded like any other 
zsh prompt.

Installation becomes:

* Place pure.zsh in `$fpath` as `prompt_pure_setup`

Usage becomes:

~~~
autoload -U promptinit
promptinit

prompt pure
~~~

It can be listed and previewed too.

I also included an Arch PKGBUILD which installs the prompt. Feel free to 
cherry-pick around that if you don't want it -- I can maintain it 
separately.

Thanks