Adding custom message support in the schema

[adilath18]

We can create custom message, by adding another property in schema i.e message for e.g.

If our schema is like this:

"schema": {
      "properties": {
        "foo": {
          "type": "integer"
        },
        "bar": {
          "type": "string"
        }
      }
    },

Then to provide custom message, we have to modify it to:

"schema": {
      "properties": {
        "foo": {
          "type": "integer"
        },
        "bar": {
          "type": "string"
        }
      },
    **"message": {
        "type" : "Invalid type"
      }**
    },

Since we are getting arguments in ValidationMessage, we can make this message more illustrative using dynamic arguments.
e.g. {0} invalid type where , {0} will be path

[stevehu]

This is a smart way to implement the customized error message with the schema. Could you please add a small doc on the feature so that other people will benefit from your work? Thanks.