[Proposal] icon

Hi @AllenFang,

It would be great to add possibility for setting an icon the current selected item (not visible in the list). My proposal would be:
- add a `iconClass` property into the `item`
- in `DropDown` add a new state `selectedItem` which would be used for rendering the title for button
- add a new optional prop `selectedItem` which would pre-fill the `selectedItem` state
- in `componentDidMount` we would check if `selectedItem` prop is set and will pre-fill the `selectedItem` state , if `title` is provided instead of `selectedItem`, we would build an item with only `text` property.

These changes would add new way how to set and render the button title without breaking old settings.

Do you agree? I can make the changes.

Martin.