(optionally) add generic docstring

Originally reported by: **Rami Abughazaleh (Bitbucket: [ramiabughazaleh](https://bitbucket.org/ramiabughazaleh), GitHub: [ramiabughazaleh](https://github.com/ramiabughazaleh))**

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Thank you for sqlacodegen 1.1.6.

I'd like to request the ability to add generic docstrings to the models/tables.

For example:

```
#!python
class Student(Base):
"""
Student
"""

class ClassRoom(Base):
"""
Class Room
"""
```

In this way, when I inspect my code using PyCharm, it doesn't produce a warning indicating that a docstring is missing for example.

Thank you

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- Bitbucket: https://bitbucket.org/agronholm/sqlacodegen/issue/25