[Loader]: `before` condition, fixes #1661

This adds a feature, `before`, that was missed in the recent `Loader` changes due to no test being written for it.

```js
YUI({
    modules: {
        "my-module": {
            condition: {
                trigger: "io",
                when: "before"
            }
        }
    }
}).use("io");
```

Granted `io` is actually used, it will ultimately produce the same effect as if you had said `io` *requires* `my-module`.

(fixes #1661)