Add new subcommand 'exec'

Hi,

I have add new subcommand. `kitchen exec`

Example usage:

```
$ bundle exec kitchen exec default -c 'whoami'
       vagrant


$ bundle exec kitchen exec default -c 'find'
       .
       ./.vbox_version
       ./cleanup.sh
       ./sshd.sh
       ./.bash_profile
       ./.bash_logout
       ./.ssh
       ./.ssh/authorized_keys
       ./.bashrc
       ./.veewee_version
       ./vagrant.sh
       ./minimize.sh
       ./.veewee_params
```