Add renderControlLabel prop

[drewbrend]

See issue: https://github.com/based-ghost/react-functional-select/issues/25

This PR adds the ability to render menu options and the selected option in the control differently.

Multi select already provided this ability via renderMultiOptions, I also needed this behaviour for a single select. I wasn't sure whether to name the prop in a similar fashion to renderMultiOptions or renderOptionLabel and went for the latter - I'm definitely open to naming suggestions 👍