Implement filters to allow for job manipulation

[tott]

Although this is likely an edge case I think this simple alteration opens up a lot of possibilities for developers. Using the changes here and a very simple test script I was able to create a simple cross blog syndication script for a multisite setup by altering the blog_id in the job_data payload as seen here https://gist.github.com/tott/e9e8e177966b478977f2129ae32aec10