XMonad.Prompt: export {selected,setCurrent,getCurrent}Completion[s]

[dcousens]

Description

I am playing with using the key bindings to modify the input, and when trying to change the input to the next [or previous] completion in the currently available completions, I needed getCurrentCompletions and selectedCompletion to be available. I have additionally added setCurrentCompletions, but if we don't want to export that, I'm happy to remove that from the PR.

I am new to Haskell, maybe I'm missing another way to do this.

Checklist

• I've read CONTRIBUTING.md

• I've considered how to best test these changes (property, unit,
  manually, ...) and concluded: N/A

• I updated the CHANGES.md file

[slotThe]

Thanks!