Add the showDiff property to geezer assertion errors

Assertion errors will now have a `showDiff` property set to true by default. This can be disabled by setting the `showDiff` config property to false, the same as can be done with standard Chai:

```js
define([ 'intern/chai!config', ... ], function (config, ...) {
    config.showDiff = false;
    ...
});
```