Allow assigning n:m relationships

Nobody has claimed this yet.

Given the example from the guides:

```crystal
class Tagging < BaseModel
table :taggings do
belongs_to tag : Tag
belongs_to post : Post
end
end

class Tag < BaseModel
table :tags do
column name : String
has_many taggings : Tagging
has_many posts : Post, through: :taggings
end
end

class Post < BaseModel
table :posts do
column title : String
has_many taggings : Tagging
has_many tags : Tag, through: :taggings
end
end
```

it would be great if there was an easy way to assign tags to posts.

For example in ActiveRecord the same example would give you a `tag_ids=` method on the `Post` model. When calling `post.tag_ids = [1, 3, 5]` ActiveRecord will transparently create and / or delete `Tagging` records to make sure the post is associated with the tags 1, 3 and 5, and only with them. This makes it easy to create HTML forms to edit relationships (either via checkboxes or multiple selects).

I am not 100% sure how this could look in LuckyRecord. Of course it would be super nice to be able to say `fillable tag_ids` in your form class and have everything else work automatically.

On the other hand: `fillable` is currently reserved for fields whose values get saved to the actual database table that belongs to the underlying model. So in some ways this would be closer to a `virtual` field :man_shrugging:

Start from the Tagging, Tag, and Post relationship example in the issue, then inspect how LuckyRecord currently handles has_many-through associations and fillable fields. The desired result is a tag_ids assignment that creates or deletes Tagging records so a post has exactly the selected tags; no files or tests are named, so the relevant entry points must first be located.

Written by the indexing model from the issue text.