Use Boris to power `wp shell`

Using the [Boris](https://github.com/d11wtq/boris/) REPL to power `wp shell` would bring several desirable traits:

* multiline support: #316 
* resilience against syntax errors: #292 

The only drawback is that Boris relies on several PHP extensions: https://github.com/d11wtq/boris/blob/master/composer.json#L5-7

Ideally, we'd let Composer check those dependencies and if they're not met, we would fall back to the existing implementation.