Adds support capistrano multistage completion

Added `xcap` because `cap` is a reserved word. `cap` completion doesn't work.

http://zsh.sourceforge.net/Doc/Release/Zsh-Modules.html#The-zsh_002fcap-Module

![terminal](https://f.cloud.github.com/assets/105652/2003945/ca08e848-8686-11e3-9167-6f8fb3dae988.gif)