add plural config make developer able to select the default collection name

I want to reach out the author of repo and check his opinion about a new feature.

## currently
this lib will use the lowercase class name as the default name in mongodb

```python
class User(Model):
name: str
```

Save this with `engine` and it will generate the collection `user` in mongodb

I also know that the lib provide a way to set custom collection name.

## propose

I want a lazy solution to just turn my class name into plural nouns and use that.

I want instead of `user`, I would like to go by the `users` without me need to specify all custom names by myself.

I know there is workaround, but i think it is nice to have this as a lib option directly.

There are many choices of interfaces that could potentially be, for example

```python

class CapitalCity(PluralMixins, Model):
pass

class CapitalCity(ModelWithPluralName):
pass

engine = AIOEngine(use_plural_collection_name=True)
```

I trusts in the pythonic instrict of author. so, I'm waiting for reply.

## how to implement

Maybe we can use this lib to help in plural conversion https://pypi.org/project/inflect/

I could do a PR.

The issue names Model, AIOEngine(use_plural_collection_name=True), and the optional inflect library; begin by tracing how the engine derives a default MongoDB collection name. A decision is still needed on the public API and pluralization behavior; done means the chosen option maps examples such as User to users without requiring per-model names.

Written by the indexing model from the issue text.