adding cache flushing

[curtismchale]

In scenarios where tools like Redis are being used the cache needs to be flushed so that the user can see the changes.

This takes the place of my previous pr: https://github.com/hijiriworld/intuitive-custom-post-order/pull/63

This still has not been tested with object caching in a Multisite environment as I don't have access to a Redis environment I can run Multisite on.