Improve query modifiers API

[Simon Yousoufov]

Issue:
Adding modifiers to the repository current requires storing them in an intermediate array and then setting that array to overwrite all modifiers:

$modifiers = $repository->getModifiers();

// Add a modifier
$modifiers[] = function (Builder $query) { /* do some modifying */ };

// Set modifiers en masse
$repository->setModifiers($modifiers);

Proposal:
The most common use case would be served by an addModifier method on the repository which accepts a Callable containing the modifier.

[Simon Yousoufov]

Fixed by #18