Add a hook for altering the indexed items

This is a hook that was curiously absent up until now. Even though items probably shouldn't be rejected by a hook, but rather by a data alteration, module might want to at least be informed when items are indexed.
Also, e.g., a module that will automatically (or optionally) prevent all unpublished nodes from being indexed would certainly have it's practical usages (as adding such a data alterations for several node indexes (if there are more) might be unnecessary work).

So, now there is the hook and people can start shooting themselves in the foot with it, if they insist. ;)