Allow support of boost (and other options) to Terms Query

Hi,

While using tire, I had problem using the boost on query terms.

I am pretty new to both tire and elasticsearch world, so maybe I am just doing it wrong.

With this indices
```ruby
class User < ActiveRecord::Base
  include Tire::Model::Search
  mapping do
    indexes :subjects do
      indexes :id, type: :integer
    end
  end
end
```

A request like : 
```ruby
search = User.search do
  query do
    boolean do
      should { 
        terms 'comments.id',  [1, 2], boost: 10
      }
    end
  end
end
```
produce such a result : 

```
curl -X GET 'http://localhost:9200/users/user/_search?load=true&size=12&pretty' -d '
  {
    "query": {
      "bool": {
        "should": [
          {
            "terms": {
              "comments.id": [
                1,2
              ]
            }
          }
        ]
      }
    },
    "size": 12
  }
'
```
When I was waiting for something like that : 
````
curl -X GET 'http://localhost:9200/users/user/_search?load=true&size=12&pretty' -d '
  {
    "query": {
      "bool": {
        "should": [
          {
            "terms": {
              "comments.id": [
                1,2
              ],
              "boost": 10
            }
          }
        ]
      }
    },
    "size": 12
  }
'
````

The attached pull request solve that.

Thank you to let me know if it helps or if you need some more informations