Add support for “:except” and “:only” options

## Except

```ruby
class User < ActiveRecord::Base
  acts_as_microscope except: [:updated_at]
end

User.created_before(2.months.ago) # works!
User.updated_before(2.months.ago) # NoMethodError
```

## Only

```ruby
class User < ActiveRecord::Base
  acts_as_microscope only: [:updated_at]
end

User.updated_before(2.months.ago) # works!
User.created_before(2.months.ago) # NoMethodError
```