Add support for field default value

[milkaknap]

Add support for default value like in django.db.models.fields.Field.
Example:

class MyModel(models.Model):
flags = BitField(flags=(...), default=('flaggy_foo', 'flaggy_bar'))

[johnboiles]

Q: have you tested this with South migrations? South uses the default parameter when creating new columns.

[milkaknap]

No, I haven't tested it with South migrations.