Add mutiple unique keys support.

Before:
```ruby
User.fetch_by_uniq_key("hooopo", :nick_name)
```

After:

```ruby
User.fetch_by_uniq_key("hooopo", :nick_name)
User.fetch_by_uniq_keys(nick_name: "hooopo")
Post.fetch_by_uniq_keys(user_id: 2, slug: "foobar")
```