add ability to set a targeting override and opt mobs in to it

Hi Lycanite,

We discussed a feature like this a few weeks ago.

I think this is probably the sanest approach that allows me to do the type of targeting changes I want on specific entities while also avoiding either neutering the AI for all mobs or having a really extensive change in implementation as many of the current targeting behaviors are dependent on private variables inside your targeting class, which means my targeting api cannot fully emulate your own targeting code atm.