Added extends plugin

The **ls** example was just to explain what i means by programs :smile: 

I started developing a vagrant plugin and then i refactored it and extracted this function to be able to reuse it, cause i saw that i will probably **entend** more programs.

This is the normal output of vagrant help
```
Usage: vagrant [options] <command> [<args>]

    -v, --version                    Print the version and exit.
    -h, --help                       Print this help.

Common commands:
     box             manages boxes: installation, removal, etc.
     connect         connect to a remotely shared Vagrant environment
     destroy         stops and deletes all traces of the vagrant machine
     global-status   outputs status Vagrant environments for this user
     halt            stops the vagrant machine
     help            shows the help for a subcommand
     init            initializes a new Vagrant environment by creating a Vagrantfile
     login           log in to Vagrant Cloud
     package         packages a running vagrant environment into a box
     plugin          manages plugins: install, uninstall, update, etc.
     provision       provisions the vagrant machine
     rdp             connects to machine via RDP
     reload          restarts vagrant machine, loads new Vagrantfile configuration
     resume          resume a suspended vagrant machine
     share           share your Vagrant environment with anyone in the world
     ssh             connects to machine via SSH
     ssh-config      outputs OpenSSH valid configuration to connect to the machine
     status          outputs status of the vagrant machine
     suspend         suspends the machine
     up              starts and provisions the vagrant environment
     version         prints current and latest Vagrant version

For help on any individual command run `vagrant COMMAND -h`

Additional subcommands are available, but are either more advanced
or not commonly used. To see all subcommands, run the command
`vagrant list-commands`.
```

And this is the "overloaded" help function from the **_extends** plugin (will be added on next commit)
```
Usage: vagrant [options] <command> [<args>]

    -v, --version                    Print the version and exit.
    -h, --help                       Print this help.

Common commands:
     box             manages boxes: installation, removal, etc.
     connect         connect to a remotely shared Vagrant environment
     destroy         stops and deletes all traces of the vagrant machine
     global-status   outputs status Vagrant environments for this user
     halt            stops the vagrant machine
     help            shows the help for a subcommand
     init            initializes a new Vagrant environment by creating a Vagrantfile
     login           log in to Vagrant Cloud
     package         packages a running vagrant environment into a box
     plugin          manages plugins: install, uninstall, update, etc.
     provision       provisions the vagrant machine
     rdp             connects to machine via RDP
     reload          restarts vagrant machine, loads new Vagrantfile configuration
     resume          resume a suspended vagrant machine
     share           share your Vagrant environment with anyone in the world
     ssh             connects to machine via SSH
     ssh-config      outputs OpenSSH valid configuration to connect to the machine
     status          outputs status of the vagrant machine
     suspend         suspends the machine
     up              starts and provisions the vagrant environment
     version         prints current and latest Vagrant version

For help on any individual command run `vagrant COMMAND -h`

Additional subcommands are available, but are either more advanced
or not commonly used. To see all subcommands, run the command
`vagrant list-commands`.


    Added automatically from the _extends plugin (_vagrant-):

        ● cd
        ● list
        ● mount
        ● umount
```

So i extended the vagrant program and added the next commands:
- cd
- list
- mount
- umount

I can now issue ```vagrant list``` and it will list a list of the configured vagrant boxes in my vagrant plugin boxes config files.
```
vagrant list
	 ● box1
```

so i can **cd**, **mount** or **umount** my vagrant boxes from any directory