[Python] Enrich convenience of ModelResult for visualization and dict result #2398

When I get results from Recognizers-Text in Python, I would like to take a quick view of what I have extracted.
In addition, if we can easily get a dict result from it, this is useful that it using the same format of converting C# object through the Protobuf.
More specifically, I will make a pull request that implements __repr__, __str__, and get_dict methods for the ModelResult class.