Add Microscope.past_participle_exceptions

Nothing better than a code sample:

```ruby
Microscope.past_participle_exceptions.merge! 'eaten' => 'eat'

class Meal
  acts_as_microscope, only: [:eaten_at]
end

meal = Meal.not_eaten.first
meal.eaten # => false

meal.eat!
meal.eaten # => true
```