success message framework

[RitoG09]

Related to: #120 and #159

• Adds support for success explanations for the required keyword.

• Adds support for negated success explanations when required appears under not.

• Introduces a helper (getPassing) that collects success explanations from keyword handlers.

Schema:
{ "not": { "required": ["a"] } }
Instance:
{ "a":1 }
Output: (now)

[
  {
    "message": "Expected a value that doesn't match the 'not' schema",
    "alternatives": [
      [
        {
          "message": "The 'a' property should not be present"
        }
      ]
    ]
  }
]

@jdesrosiers If this direction looks reasonable, my plan is to extend the same mechanism to additional keywords. I need feedback on this and also request the order that i should match to implement it for all required keywords..

[jdesrosiers]

I don't think a "mode" flag is going to work well in at least some cases. I think we need a whole new passing handler function. But, the negated flag on the passing handler I think should work.

Also, please make sure to run the tests and linter before submitting code.