add DictField
You can use the `PickledField`, added just now:

https://github.com/coleifer/peewee/blob/master/playhouse/fields.py#L293-L302

If you're using a new-ish version of Postgres you can use the JSON field or the HStore field.

If you're using a new-ish version of SQLite with the JSON1 extension compiled in or loadable, you can use the JSONField from the SQLite extension module.

Hello!
Is it possible to implement DictField (basing for example TextField)?
I need to store unstructed data in model and I think such feature would be useful.
