Extend Assert#pushResult to receive a custom diff value

As requested by @wycats, it would be good to extend Assert#pushResult to receive a custom diff value, where it does not replace the default QUnit.diff (still useful for other assertions) but it prints a customized diff value.

Examples:

// Using default diff
QUnit.assert.prototype.customAssertion1 = function(actual, expected, result = actual == expected) {
  return this.pushResult({result, actual, expected});
};

// Using custom diff
QUnit.assert.prototype.customAssertion2 = function(actual, expected, result = actual == expected) {
  return this.pushResult({
    result,
    diff: lineDiff(actual, expected)
  });
};

If diff is given, QUnit should get its value and ignore calling the default QUnit.diff.

Start at Assert#pushResult and trace how it currently obtains the default QUnit.diff value. Confirm how a supplied diff should take precedence while leaving the default available to other assertions, then add coverage for both the default and custom cases using the examples in the issue as the completion criteria.