Propel2 console completion plugin

This plugin provides the possibility of completion commands and arguments the Propel2 CLI.
![Screenshot with command completion for Propel2 commands](https://cloud.githubusercontent.com/assets/1755934/3103051/c28dd500-e64f-11e3-8a8a-1d8f89823c30.png)
P.S. I had to create a new branch for this plugin. I need to do it or you accept the PR?