added possibility to filter terms with execution: and option 

let's say i have a many-to-many relationship between `Project` and `Category` models.

i defined  `Project` Model like this in order to get categories_title:
```ruby
class Project < ActiveRecord::Base
  def search_data
    attributes.merge(
      categories_id: categories.map(&:title)
     )
  end
end
```

so in controller:

```ruby
 @project_search = Project.search 'house', where: { categories_title: ['farm'] } 
```

results in view:

```ruby
 # sidebar 
 farm (3)
 house (1)
 apartment(2)
```

when i click in `house` i expected just one __project result__, but it returned 4.

i'm not very sure if i fit to your code conventions, but i fixed this way when i'm doing this kind of search:

```ruby
 @project_search = Project.search 'house', where: { categories_title: { and: ['farm', 'house'] } }
```
and it returned just one project as i expected.