#1287 - Support ManyToManyField in model_to_dict
I've merged a patch that adds support manytomany in model_to_dict. Available in Peewee 3.x.

http://docs.peewee-orm.com/en/latest/peewee/playhouse.html#model_to_dict
Adds `dereference_many_to_many_fields` parameter to `model_to_dict` function.

This parameter controls whether to hide the junction table for `ManyToManyFields`.
See the related issue for details.

The default value is False, which preserves the old behavior for compatibility.

I've also added some simple tests for the new behavior; Let me know if they are insufficient and/or something should be changed.