TypeScript support?

[Yaojian]

Great lib, and it will be even greater provides the typescript .d.ts ^_^.

[danzel]

These should get you started, feel free to add them.

The propTypes say that buttons is required, but then they are supplied in defaultProps, so I've made them optional here.

declare module 'react-confirm-alert' {

  export interface ReactConfirmAlertProps {
    title?: string;
    message?: string;
    buttons?: Array<{
      label: string;
      onClick: () => void;
    }>;
    childrenElement?: () => React.ReactNode;
    customUI?: (customUiOptions: { title: string, message: string, onClose: () => void }) => React.ReactNode;
    willUnmount?: () => void;
  }

  export function confirmAlert(options: ReactConfirmAlertProps): void;

  export default class ReactConfirmAlert extends React.Component<ReactConfirmAlertProps>{
  }
}

[GA-MO]

@danzel Can you make this PR?