breaking change: adjust how models are selected

[jamesturk]

probably going to introduce a breaking change in 0.6 or 0.7 to how models are selected

Now that OpenAI is releasing different versions of models (3.5-turbo has 4 current versions between the token limit and the different iterations) I think model configuration/fallback needs to change a bit.

If 3.5 parsed the results and didn't do well, the next fallback should be 4, if the token limit was exceeded however it should go to 3.5-16k. There are a lot of possible conditions for this, and people that want 100% control can explicitly pass a single model, but the way the fallback chain is traversed can improve to reduce redundant requests.

Something like:

models=[GPT35T(allow_16k=True), GPT4(allow_32k=False)]

This would try gpt-3.5-turbo only once, either at 4k or 16k based on input.
Then it would try gpt-4.
Selecting particular revisions could work this way as well.

Probably makes the most sense to do this as part of #18