Simplify subscribing to feeds

If you're using Brief as your feed reader, it is unintuitive to having to choose Live Bookmarks as the subscription method and select the bookmark folder every time.

This is not to say Brief won't integrate with Live Bookmarks anymore. Rather, it Brief should have its own subscription method that automatically creates a Live Bookmark in Brief's home folder.

Note: this will make it less obvious to users that Brief integrates with Live Bookmarks and that you can create subfolders by using the Library. Therefore, it this relies on issue #14.