Dictionary field

[beregond]

Dictionary (schemaless) field is first most needed field for jsonmodels:

• Added support for schemaless/empty {'type': 'object'} fields. #84
• Added new field: DictField #141

And there are good reasons to having one - for example we could have a list of tags like {"tag1": "value1", "tag2": 123, ..., "tagn": None}.

(This is part of bigger problem -> #143)

The question is how to implement it - we can choose either of those two pull requests as base, or came up with something different.

Things to consider:

• how deep the constraints should go? We can either declare 'object', or some valid types, or even TypedDict?