Many-to-many relation support

Example usage:


```ruby
class Link
  include NoBrainer::Document
  field :url
  belongs_to :comment
end


class Comment
  include NoBrainer::Document

  field :body
  has_many :posts, through: 'comments_posts'

  has_many :links
end

class Post
  include NoBrainer::Document

  field :title
  field :content
  field :author

  has_many :comments, through: 'comments_posts'
end


class CommentsPost
  include NoBrainer::Document

  belongs_to :comment
  belongs_to :post

  field :category
end

p = Post.create(title: 'Test Article', content: 'Text Text Text Text Text', author: 'Alex')
p.comments(category: 'Art').create(body: 'Text Text Text Text Text')
p.comments(category: 'Internet').create(body: 'Text Text Text Text Text')


# Print comments from 'Art' category
puts p.comments(category: 'Art').to_a.inspect

# Print comments from 'Internet' category
puts p.comments(category: 'Internet').to_a.inspect

# Print all comments
puts p.comments.to_a.inspect
```