Media widget

Based on #1.

Usage:
https://github.com/SparkartGroupInc/jungle-solidus/wiki/Integration
https://github.com/SparkartGroupInc/jungle-solidus/wiki/Widgets#media
@pushred I tracked this weird browse page condition to a [2007 bugfix](https://github.com/SparkartGroupInc/jungle/commit/e5dd484d048ed16177d5fe435cd6f0476f28843a). It was probably due to a site that had no display page for videos, or maybe audios? Do you think I can remove this condition, assuming a display page will always exist? Maybe audio browse pages don't need display pages in general?

In short, I want to remove all those `defaultFor` for the display and browse links, they're all over the place, in all widgets. So I'd like to automatically assign a browse and display page [in the helper](https://github.com/SparkartGroupInc/jungle-solidus/blob/media-widget/helpers/index.js#L10), something like
```javascript
options.hash.browse_page || (options.hash.browse_page = this.site.pages.browse[widget] || widget + 's');
```

But doing so will break the media widget condition shown here...