allow meta to be assigned to an autocomplete dropdown

Pretty simple little adjustment. I am using the auto complete in a situation where I want to use the meta as a way to correlate the result by id rather then just using the freeform text field as my UID.

This tweak just allows you to pass `['', {}]` to the auto complete result callback, 0 is the string, 1 is the meta.

let me know if this appeals and I'll update this PR with tests and docs.