Dropdown: Allow function in optionLabel

[lukascernydis]

Hallo,
I would like to propose a change in Dropdown component, specifically in optionLabel signature to allow functions.

Currently, if you try to use function for label formating, it is functional, but typescript compiler will complain about No overload matches this call..

<Dropdown
    optionLabel={x => `${x.number} - ${x.name}`}
/>

[melloware]

Feel free to open a ticket and submit a PR.