Auth - parameter to override db table name

user is reserved keyword in postgres. Every postgres who want to use Auth will have to add meta class in custom model as per https://github.com/coleifer/flask-peewee/issues/14

```python
class Meta:
        db_table = 'users' # Postgres reserves user as a keyword
```

IMO, this should be made part of Auth with ability to user to customize db tablename.

```python
class Meta:
    db_table = self.db_table  # Postgres reserves user as a keyword
```
where db_table can be passed to Auth i.e. 
```python
auth = Auth(app, db, db_table='auth_user')
```