Add support for .borisrc file, Boris::setPrompt

Personally I find it useful to keep my user-specific configurations in my `$HOME`, without having to much around with proxy scripts if I don't absolutely have to.

This PR adds support for:

- Reading a `.borisrc` file, by default from the user's `$HOME`, or the local `cwd`
- Setting the prompt through a `Boris::setPrompt` method (currently only supported in the constructor).

Example `.borisrc` file:

```php
<?php
$boris->setPrompt('bananas> ');
$boris->setLocal(array('foo' => 'bar'));

# etc...
```

I kept it purposefully simple for now, there's a lot of room to play around with this if you'd like.

Cheers! :smile: