adding support for custom filter and combine when generating seed urls

Trello:
https://trello.com/c/uZnI5ehS/184-express-yui-allow-customization-of-filter-and-combine-for-the-seed-per-request

This feature covers a very edge case where someone want to force the filter and the combine flag for the seed urls. For example, if you have a middleware to detect debug mode, you might want to customize that for a particular request by creating a middleware that exposes custom values. Here is an example of the debug middleware:

```
function debugMiddleware(req, res, next) {
    res.expose('debug', 'window.YUI_config.filter');
    res.expose(false, 'window.YUI_config.combine');
    res.expose(req.app.yui.getSeedUrls({
         filter: 'debug',
         combine: false
    }), 'window.YUI_config.seed');
    next();
}
```