Add clear/confirm action handlers, add button text props

[veax]

Hello, thanks for this lib, I think it will be useful to add button to confirm selection (for exemple in my case I show this component as popup on overlay and I wanted to click OK button to close popup). It's also related to #12

So this PR add:

• 2 optional handlers (onConfirm/onClear) executed when pressing OK/Clear buttons respectively
• 2 props to be able to change text of buttons (#13). Also in some cases, we maybe don't want to show button at all (like only clear button actually). This is possible if empty string passed as prop

• I also updated README file respectively

Thanks for your attention

[dilipchandima]

@veax merged and published as v3.3.0

Can you please check if it is working fine with your app? and update here :)

[veax]

@dilipchandima yes, it's works as expected, thanks for merge :)