Add information about the cost of the API call

I think it is nice to be transparent about how much is each text generation costing. I suggest to add some calculation about how much has the user paid for each command (PR generation for now)

[jimzer]

That's a good point, thanks.

Actually I just released a new feature to generate README files for repository and it computes the amount of tokens it will need and ask the use to confirm.

We could add the same mechanism for the pr generate command?