Allow site developers to customize FeedHit creation

[jacobwegner]

https://github.com/pinax/pinax-blog/blob/be8dfb29c5d8d5a74c9941901a5fe0725b4a9a24/pinax/blog/views.py#L205

On long-running sites with pinax-blog, the FeedHit table can grow quite large.

Consider adding hookset support to override FeedHit recording, or possibly a management command similar to clearsessions