Enable use of generators without engine

[kozlek]

sqlacodegen is a great tool that does two things when used as cli:

1. generate a MetaData object using SQLAlchemy built-in reflection
2. generate code using the generated MetaData from step 1

While most of users use the CLI, the core value of sqlacodegen is the code generator.

This PR eases the standalone usage of the code generator by removing the need of an engine (and the associated driver). Actually, the engine+driver is only needed for the reflection (step 1), while a dialect is enough for step 2.

Example usage

from sqlacodegen.generators import DeclarativeGenerator
from sqlalchemy import Dialect
from sqlalchemy.dialects import postgresql

metadata = sa.MetaData()
generator = DeclarativeGenerator(metadata, dialect=postgresql.dialect(), options=set())
print(generator.generate())

[coveralls]

coverage: 97.02% (-0.005%) from 97.025%
when pulling 9a2e003 on kozlek:remove-engine-dependency
into e776bb7 on agronholm:master.