Added extends plugin

By program I mean program :smile:

For instance it could be **ls** or **ping** but also **vagrant** (like in my case) or maybe somebody would like to add commands to **rabbitmqctl** without knowing erlang or they are just too simple (aliases / functions) to be coded in erlang.

What you accomplish with this is the ability to invoke you functions via the program that you are expending.

Hope that I expressed myself correctly :smile: