Config additions: "Cascade" & "Mandatory"

[trepagnier]

I added a couple of features to the form that can be modified in the "fields" object within the default config:

cascade

"cascade" controls the visibility of fields in the form. Any questions that follow a true/false radio field can be hidden using the following arguments:

• "affirmative", if all fields should disappear if the response is "Yes".
• "negative", if all fields should disappear if the response is "No".

  mandatory

"mandatory": true will make the field mandatory if it is visible. If it is hidden by the cascade, then the 'mandatory' class will be removed.

[driskull]

hey @darinacosta I think we're going to have to do something different at a later time to handle a larger use case where there's more of a decision tree. Thanks for the commit though!