Add `loadAll` method

[timacdonald]

Introduces a loadAll method to make loading all defined features easier.

- Feature::for($user)->load(['feature-1', 'feature-2']);
+ Feature::for($user)->loadAll();

// or

- Feature::for($user)->load(Feature::defined());
+ Feature::for($user)->loadAll();

I don't feel this needs a loadAllMissing variant as this would likely be the first thing you do in your stack.