Add config.define option

This commit adds support for a `define` requirejs config option. This option enables pre-defining module values at config time.

```javascript
requirejs.config({
    define: {
        fooModule: { foo: "bar" }
    }
});
```

This is a very simple implementation. It mainly addresses the need I had to pre-define module in requirejs context to use as a namespaced/sandboxed environment as described in [this thread](https://groups.google.com/forum/#!topic/requirejs/TYxjcTLCISw).