Make configured functions available in the Twig views.

For your consideration, based on https://github.com/rcrowe/TwigBridge/issues/42

This allows the user to specify functions (for instance laravel helpers) that should be made available to the Twig Views.

```php
'functions' => array(
    'base_path',
    'app_path',
    'camel_case',
)
```

This configuration would make these three functions available to the views as:

```php
{{ base_path() }}
{{ app_path() }}
{{ camel_case("snake_case_incoming") }}
```

Also possible to specify a Closure callback directly:

```php
'functions' => array(
    'bond' => function($name, $lastname) {
        return "My name is {$lastname}, {$name} {$lastname}...";
    },
)
```

And later in the view:

```php
{{ bond("Rob", "Crowe") }}
```
For a dramatic "My name is Crowe, Rob Crowe...".