Add the auto_now parameter to the TimestampField field
You can already do this by overloading the default datetime class, something like:
``` python
from datetime import datetime

import peewee as pw


# pylint: disable=locally-disabled, invalid-name
def NowDateTimeField(**kwargs):
    """ A peewee DateTimeField which defaults to 'now'
    """
    constraints = kwargs.pop('constraints', [])
    if not kwargs.get('null'): # also default to 'now' on insert
        constraints.append(pw.SQL('DEFAULT CURRENT_TIMESTAMP'))

    return pw.DateTimeField(
        default=datetime.now,
        constraints=constraints,
        **kwargs,
    )

``` There are a couple options:

* Specify default=datetime.datetime.now (no parens!!)
* Override the save() method
* Use a constraint
* Create a trigger to set new value on update
I want to add this parameter, when set to True, the database entry will be updated and changed to the current time