Port the oh-my-zsh cloud theme to prezto.

I recently switched from oh-my-zsh to prezto. I love it, but I missed my old theme, so I ported it over. Maybe y'all will have a use for it too. 

This is a port of oh-my-zsh's [cloud theme](https://github.com/robbyrussell/oh-my-zsh/blob/master/themes/cloud.zsh-theme) to prezto. The prompts are the same as the omz version, but instead of defining the prompt's prefix in the `ZSH_THEME_CLOUD_PREFIX` environment variable it's done via the config. That seemed like a more prezto-ish way to do it. Hope y'all like it.

The original oh-my-zsh version:
![Original Screenshot](https://camo.githubusercontent.com/97f24b7ff6670393f4414d81532e1431ef4fbe7c/687474703a2f2f662e636c2e6c792f6974656d732f31346635383037653838373131323264396665302f636c6f75642e706e67 "Original Screenshot")

The ported prezto version:
![Ported Screenshot](http://i.imgur.com/mJCZ8rE.png "Ported Screenshot")
Indent everything bellow Usage by two spaces; so, the heading is visible.