Add method for defining custom assertions

[jzaefferer]

We currently suggest adding function properties to QUnit.assert, though considering the plans in #374, this might actually be a bad idea. If we make the assert argument an instance with a prototype, we may want to remove the global QUnit.assert and instead offer a method for defining custom assertions. Can be a very simple signature like this:

QUnit.addMethod( "closeEnough", function( ... ) {
  QUnit.push( ... );
});

We really need a prototype for #374 to see if that is feasible and evaluate this and some other discussions.

[leobalter]

Closely related to #533

[JamesMGreene]

@leobalter: Not that closely related, I'd say.

#533 is more about composing/aggregating assertions, whereas this one is simply about introducing an API abstraction for adding new assertions to the Assert prototype.

I'm not sure that this is truly necessary as we can just assume that QUnit.assert should be used as the prototype when creating a new Assert instance to pass to each test context, right?

[leobalter]

Yes, and extending QUnit.assert allows the method to call this.push( condition, actual, expected, message );

[jzaefferer]

Agreed, extending QUnit.assert still works fine, and with this.push was improved as well.