Improved extension interface

c.f. 8fc41d0, #1634 & #1635

**NOTE** requires a `composer update` in Bolt's root
Just tried:
```php
$filename = 'test.txt';
if (is_readable($filename)) {
    echo 'The file is readable';
} else {
    echo 'The file is not readable';
}
```

With a file called Test.txt in the same directory on my Linux laptop and it returns `The file is not readable`