First draft command

@westonruter Here is the first draft for the command wp cli ssh.

One can invoke `wp ssh core version --host=vagrant` and it will forward to the specified host.

We can also create an alias for wp to forward by default to vagrant:
```bash
alias wpssh = 'wp ssh --host=vagrant'
```
I used a reflection class to prevent having copy-pasted core function. This should be temporary. 

This should close #4.