Adds options.transformRequest to allow changing the structure of the request

[Jendoliver]

(Defaults to previous implementation)

I just needed to do this to use an endpoint which was expecting the autocomplete search term in a nested object (something like the example I added to the docs). I agree that this is pretty strange but as modifying the endpoint is not an option and I think this could be the case for another person, my opinion is that this could be helpful.

I know there's a PR with a similar name (https://github.com/devbridge/jQuery-Autocomplete/pull/179) but that one only changes the search term instead of changing the request structure.

If there's already a way to do this then sorry for not being able to find it!