choices just like django or mongoengine
i'm on the fence about this, should they be implemented as a check constraint or solely in python?  if you assign something that is an invalid choice, should it error out?  if so, what type of error?
 i checked django, it seems you can assign invalid values to a field that specifies `choices`.  you can also save these invalid choices.  probably just for modelform validation.  so, options:
- db-level check constraint -- bad because if you want to change your choices that means schema change
- python-level on save
- python-level on assignment
- none at all, just for metadata and other libraries (flask-peewee, wtf-peewee)

I'm leaning towards none at all, same as django.
 Maybe it should output a warning in DEBUG mode
 I've added this in commit 2d076f069baf715d6dfd0070df861f0b3f417724

I've also added integration in the wtf-peewee bindings (wtforms form library integration)
https://github.com/coleifer/wtf-peewee/commits/master

I've also added a small example to the flask-peewee app:
https://github.com/coleifer/flask-peewee/commit/225f85d68fe39c0271b7a1a00b55ede1ede7f41f

Finally, there's no debug mode per se with peewee, so there's no warning

Add choices to fields like django and mongoengine.
