Change extra installations to specific drivers

[aminalaee]

Closes #429 and closes #421.

Changes extra installations from:

pip install databases[postgresql+asyncpg]

To:

pip install databases[asyncpg]

This will be a step towards downplaying the default drivers.
Even though it keeps default drivers to work like pip install databases[postgresql] but will remove them from the docs to point to specific drivers.

And also worth mentioning that this will be the API, still mentioned the default in the docs:

database = Database('sqlite:///example.db')

This is equivalent to:

database = Database('sqlite+aiosqlite:///example.db')

I think we should also downplay the default dialect and specify the driver in the docs.

[aminalaee]

@collerek Do you think you can review this?

[aminalaee]

@collerek thanks for the review. that makes sense.
I'll also update documentation for the database URI to downplay the idea of default drivers:

So instead of this:

database = Database('sqlite:///example.db')

We would use this in the docs:

database = Database('sqlite+aiosqlite:///example.db')

Even though both will work for now, but we wouldn't encourage the default drivers both in installation and usage.