update Dropdown to accept optionHeight prop

[mmartinsky]

Closes #544

Add the ability to set the optionHeight of the react-virtualized-select options. This is useful if you have labels that are too long to fit on a single line, but don't want the options to overlap.

New, using optionHeight={50}

Old/Default:

[alexcjohnson]

Thanks @mmartinsky - this will be helpful. (For the sake of others reading this, I'll note that 35 is the default from the underlying component https://github.com/bvaughn/react-virtualized-select. It makes sense to list this explicitly here so people know the baseline against which to provide a different value)