[* Choice] ChoiceRecognizer support for "maybe" result

[hansmbakker]

Is your feature request related to a problem? Please describe.
I have cases where a user might say something like "I'm not sure" besides "Yes" or "No". I believe the ChoiceRecognizer currently only has support for true and false. I would like to handle this "I'm not sure" response as well.

Describe the solution you'd like
Have a recognizer in Recognizers-Text that handles the "I'm not sure" situation just like you can have a Nullable<bool> type.

Describe alternatives you've considered

• Create and train a LUIS app that returns the responses as intents
• Implement this in a chatbot using Suggestion buttons and perform string matching