[WIP] Allow Lotus::Model to register adapter instance

Related to point 1 of lotus/model#33

The idea is to have a class attribute `adapters` which is a hash for adapter config templates or adapter instances container.

/cc @unrealhoang I am going to start from scratch
I understand the intent, but it doesn't cover the behavior.
If I have the following implementation, the test pass, but I'm not spec'ing nothing valuable here.

```ruby
def configure(&blk)
  # empty implementation, it will raise RuntimeError and the test will pass
end
```

What if we test like this:

```ruby
Lotus::Model.configure do
  adapter :sql, 'postgres://localhost/database', default: true
end

adapter = Lotus::Model.configuration.adapters.fetch(:sql)
adapter # ...
```

This test ensures **both** that the block is yielded and the method has a meaningful implementation.