Add post process hook performed after cache for filtering

Currently, if a filter allows for dynamic data, it must specify 'no cache' to true in order to prevent its data from becoming stale. While this is fine for fairly lightweight filter chains, combining dynamic data with some heavy static crunching becomes impossible with this setup. For example:

1. Expensive Static Filter
2. Cheap Dynamic Filter

After step 2, we cannot cache, as the dynamic filter will cause problems.

However, if we inserted the cache in-between these two filters:

1. Expensive Static Filter
2. Cache
3. Cheap Dynamic Filter

We would reap the benefits of both worlds.

Thus, we add a 'post process' $op for hook_filter() which is always run, either on freshly generated text or text retrieved from the database. Patch is attached.

Comments? Naming problems? Should be another hook function? This won't get into 6.x, maybe for 7.x it would be nice. Patch attached.