SQLAlchemy filter operators

[aminalaee]

Closes #14.

Allows usage of SQLAlchemy filter methods in the filter query:

notes = await Note.objects.filter(id__in=[1, 2, 3]).all()

Is the same as:

notes = await Note.objects.filter(Note.columns.id.in_([1, 2, 3])).all()

I also added a shortcut property Model.columns to access SQLAlchemy columns easier. Pretty basic, but should be enough for now.